Bluetooth Low Energy (Bluetooth LE, colloquially BLE, formerly marketed as Bluetooth Smart1) is a wireless personal area network technology designed and marketed by the Bluetooth Special Interest Group (Bluetooth SIG)2 aimed at novel applications in the healthcare, fitness, beacon,3 security, and home entertainment industries.4 Compared to Bluetooth Classic, Bluetooth Low Energy is intended to provide considerably reduced power consumption and cost while maintaining a similar communication range.
BLE and Bluetooth Classic use different sets of radio frequencies, and BLE is, therefore, independent of classic Bluetooth, and has no direct compatibility. However, Bluetooth Basic Rate/Enhanced Data Rate (BR/EDR) and BLE can coexist. The original specification was developed by Nokia in 2006 under the name Wibree,5 which was integrated into Bluetooth 4.0 in December 2009 as Bluetooth Low Energy.
Mobile operating systems including iOS, , and , as well as desktop computer operating systems including , , , and , natively support Bluetooth Low Energy.
Where Bluetooth Low Energy leads. Every connection below has a source: a link in this article, a shared Wikipedia category, or CirrusSearch text similarity. Articles already on your trail are left out.
Bluetooth Low Energy (Bluetooth LE, colloquially BLE, formerly marketed as Bluetooth Smart) is a wireless personal area network technology designed and marketed by the Bluetooth Special Interest Group (Bluetooth SIG) aimed at novel applications in the healthcare, fitness, beacon, security, and home entertainment ind…
BLE
Bluetooth Low Energy (Bluetooth LE, colloquially BLE, formerly marketed as Bluetooth Smart) is a wireless personal area network technology designed and marketed by the Bluetooth Special Interest Group (Bluetooth SIG) aimed at novel applications in the healthcare, fitness, beacon, security, and home entertainment ind…
Bluetooth Smart
Bluetooth Low Energy (Bluetooth LE, colloquially BLE, formerly marketed as Bluetooth Smart) is a wireless personal area network technology designed and marketed by the Bluetooth Special Interest Group (Bluetooth SIG) aimed at novel applications in the healthcare, fitness, beacon, security, and home entertainment ind…
Services may also include other services as sub-functions; the main functions of the device are so-called primary services, and the auxiliary functions they refer to are secondary services.
Bluetooth Low Energy is distinct from the previous (often called "classic") Bluetooth Basic Rate/Enhanced Data Rate (BR/EDR) protocol, but the two protocols can both be supported by one device: the Bluetooth 4.0 specification permits devices to implement either or both of the LE and BR/EDR systems.
In 2011, the Bluetooth SIG announced the Bluetooth Smart logo so as to clarify compatibility between the new low-energy devices and other Bluetooth devices.7
Bluetooth Smart Ready indicates a dual-mode device compatible with both classic and low-energy peripherals.8
Bluetooth Smart indicates a low-energy–only device which requires either a Smart Ready or another Bluetooth Smart device in order to function.
With the May 2016 Bluetooth SIG branding information, the Bluetooth SIG began phasing out the Bluetooth Smart and Bluetooth Smart Ready logos and word marks and reverted to using the Bluetooth logo and word mark9 along with a new blue color.
Target market
The Bluetooth SIG identifies a number of markets for low-energy technology, particularly in the smart home, health, sport, and fitness sectors.10 Cited advantages include:
low power requirements, operating for "months or years" on a button cell.
small size and low cost.
compatibility with a large installed base of mobile phones, tablets, and computers.
History
The now-defunct Wibree logo
In 2001, researchers at Nokia determined various scenarios that contemporary wireless technologies did not address.11 The company began developing a wireless technology adapted from the Bluetooth standard which would provide lower power usage and cost while minimizing its differences from Bluetooth technology. The results were published in 2004 using the name Bluetooth Low End Extension.12
After further development with partners, in particular Logitech and within the European project MIMOSA,a and actively promoted and supported by STMicroelectronics since its early stage,b the technology was released to the public in October 2006 with the brand name Wibree.15 After negotiations with Bluetooth SIG members, an agreement was reached in June 2007 to include Wibree in a future Bluetooth specification as a Bluetooth ultra-low-power technology.1617
The technology was marketed as Bluetooth Smart and integration into version 4.0 of the Core Specification was completed in early 2010.18 The first smartphone to implement the 4.0 specification was the iPhone 4S, released in October 2011.19 A number of other manufacturers released Bluetooth Low Energy Ready devices in 2012.
Applications
Screenshot of WebAuthn Hybrid Transport with Bluetooth Low Energy (known as cloud-assisted Bluetooth Low Energy, caBLE) on Chromium25
Borrowing from the original Bluetooth specification, the Bluetooth SIG defines several profiles – specifications for how a device works in a particular application – for low-energy devices. Manufacturers are expected to implement the appropriate specifications for their device in order to ensure compatibility. A device may contain implementations of multiple profiles.
The majority of current Low Energy application profiles are based on the Generic Attribute Profile (GATT), a general specification for sending and receiving short pieces of data, known as attributes, over a low-energy link.26 The Bluetooth mesh profile is an exception to this rule, being based on the General Access Profile (GAP).27
Mesh profiles
Bluetooth mesh profiles use Bluetooth Low Energy to communicate with other Bluetooth Low Energy devices in the network. Each device can pass the information forward to other Bluetooth Low Energy devices, creating a "mesh" effect – for example, switching off an entire building of lights from a single smartphone.28
MMDL () – for application layer definitions. The term "model" is used in mesh specifications instead of "profile" to avoid ambiguities.
Implementation
Chip
Starting in late 2009, Bluetooth Low Energy integrated circuits were announced by a number of manufacturers. These ICs commonly use software radio, so updates to the specification can be accommodated through a firmware upgrade.
Hardware
Current mobile devices are commonly released with hardware and software support for both classic Bluetooth and Bluetooth Low Energy.
Windows 8 and later57 (Windows 7 and earlier requires drivers from Bluetooth radio manufacturer supporting BLE stack as it has no built-in generic BLE drivers.58)
Android 4.3 and later.59 Android 6 or later requires location permission to connect to BLE.
Bluetooth Low Energy technology operates in the same spectrum range (the 2.400–2.4835 GHz ISM band) as classic Bluetooth technology, but uses a different set of channels. Instead of the classic Bluetooth 79 1-MHz channels, Bluetooth Low Energy has 40 2-MHz channels. Within a channel, data is transmitted using Gaussian frequency shift modulation, similar to classic Bluetooth's Basic Rate scheme. The bit rate is 1 Mbit/s (with an option of 2 Mbit/s in Bluetooth 5), and the maximum transmit power is 10 mW (100 mW in Bluetooth 5). Further details are given in Volume 6 Part A (Physical Layer Specification) of the Bluetooth Core Specification V4.0.
Bluetooth Low Energy uses frequency hopping to counteract narrowband interference problems. Classic Bluetooth also uses frequency hopping but the details are different; as a result, while both FCC and ETSI classify Bluetooth technology as an FHSS scheme, Bluetooth Low Energy is classified as a system using digital modulation techniques or a direct-sequence spread spectrum.63
The Bluetooth SIG officially unveiled Bluetooth 5 on 16 June 2016 during a media event in London. One change on the marketing side was that the point number was dropped, so it is now just called Bluetooth 5 (and not Bluetooth 5.0 or 5.0 LE like for Bluetooth 4.0). This decision was made to "simplify marketing, and communicate user benefits more effectively".20 On the technical side, Bluetooth 5 will quadruple the range by using increased transmit power or coded physical layer, double the speed by using optional half of the symbol time compared to Bluetooth 4.x, and provide an eight-fold increase in data broadcasting capacity by increasing the advertising data length of low-energy Bluetooth transmissions compared to Bluetooth 4.x, which could be important for IoT applications where nodes are connected throughout a whole house.21 An "advertising packet" in Bluetooth parlance is the information that is exchanged between two devices before pairing, i.e. when they are not connected. For example, advertising packets allow a device to display to the user the name of another Bluetooth device before pairing with it.22 Bluetooth 5 increased the data length of this advertising packet. The length of this packet in Bluetooth 4.x was 31 bytes (for broadcast topology).
The Bluetooth SIG officially released Mesh Profile and Mesh Model specifications on 18 July 2017. Mesh specification enables using Bluetooth Low Energy for many-to-many device communications for home automation, sensor networks, and other applications, where sensors, robots, smartphones, and other equipment need to be coordinated.2324
Mesh models
Health care profiles
There are many profiles for Bluetooth Low Energy devices in healthcare applications. The Continua Health Alliance consortium promotes these in cooperation with the Bluetooth SIG:
BLP (Blood Pressure Profile) – for blood pressure measurement
HTP (Health Thermometer Profile) – for medical temperature-measurement devices
"Electronic leash" applications are suited to the long battery life possible for "always-on" devices.29 Manufacturers of iBeacon devices implement the appropriate specifications for their device to make use of proximity-sensing capabilities supported by Apple'siOS devices.30
Relevant application profiles include:
FMP (the "find me" profile) – allows one device to issue an alert on a second misplaced device.31
PXP (the proximity profile) – allows a proximity monitor to detect whether a proximity reporter is within a close range. Physical proximity can be estimated using the radio receiver's RSSI value, although this does not have absolute calibration of distances. Typically, an alarm may be sounded when the distance between the devices exceeds a set threshold.
An example of proximity sensing usage is WebAuthn Hybrid Transport with cloud-assisted Bluetooth Low Energy (caBLE), allowing roaming authenticators such as smartphones to authenticate with WebAuthn credential (known as passkey). Bluetooth Low Energy is used as a proximity check between roaming authenticators and clients (such as personal computers).25
Alerts and time profiles
The phone alert status profile and alert notification profile allow a client device to receive notifications such as incoming call alerts from another device.
The time profile allows current time and time zone information on a client device to be set from a server device, such as between a wristwatch and a mobile phone's network time.
Battery
The Battery Service exposes the Battery State and Battery Level of a single battery or set of batteries in a device.
Audio
Announced in January 2020, LE Audio is built on top of Bluetooth 5.2 / 5.3. It allows the Bluetooth LE protocol to carry audio, with lower battery consumption compared to regular Bluetooth Audio;32 features like one set of headphones connecting to multiple audio sources or multiple headphones connecting to one source (known as Auracast);333435 and support for hearing aids.36 It introduces LC3 as its default codec.32 The standard has a lower minimum latency claim of 20–30 ms vs. Bluetooth Classic audio's 100–200 ms.37
Specifications on the implementation of Basic Audio Profile and Coordinated Set Identification was released in 2021,38394041 and the Common Audio Profile and Service in March 2022.4243 On 12 July 2022, the Bluetooth SIG announced the completion of Bluetooth LE Audio.44
Auracast implements public audio broadcasting services, allowing multiple receivers to connect to a single audio transmitter without going through Bluetooth device pairing first. It is a trademark owned by the Bluetooth SIG.
Auracast transmitters use wireless advertisements to broadcast information about available streams – such as transmitter name, type of audio, program title, and language – which can be displayed on the receiver and in the apps on smartphones and smartwatches, or processed by the receiver to automatically connect to preferred sources.51
Bluetooth LE–enabled smartphones function as an assistant to control Auracast receivers like headphones and earbuds. Broadcast Audio URI, marketed as "Scan to Listen", provides information required to connect to the specific audio stream, encoded with a QR code or transmitted using NFC. This allows users to directly select a specific audio stream in applications such as audio guides in a museum, simultaneous translations in multiple languages, silent TV screens in a bar or community center, public events or music performances at conference halls / sports venues / houses of worship, station and service announcements in public transport, and flight announcements in airports.52
MIDI
Developed by the MIDI Manufacturers Association (MMA) Working Group (now The MIDI Association), there is a formalized Bluetooth LE MIDI (BLE-MIDI) specification,53 a wireless protocol designed to transmit MIDI data over Bluetooth Low Energy. It focuses on power-efficient wireless MIDI communication for devices such as MIDI controllers, keyboards, pedals, etc., that don't continuously stream data – and without significant latency. Operating systems that support it include Windows 10 Version 1607, macOS and Android.
In December 2020, the Bluetooth SIG released a draft specification for a wearable exposure-notification service. This service allows exposure-notification services on wearable devices to communicate with, and be controlled by, client devices such as smartphones.54
Mobile phones, gaming, headsets, stereo audio streaming, smart homes, wearables, automotive, PCs, security, proximity, healthcare, sports & fitness, etc.
Mobile phones, gaming, smart homes, wearables, automotive, PCs, security, proximity, healthcare, sports & fitness, industrial, etc.
More technical details may be obtained from official specification as published by the Bluetooth SIG. Note that power consumption is not part of the Bluetooth specification.
Advertising and discovery
BLE devices are detected through a procedure based on broadcasting advertising packets. This is done using 3 separate channels (frequencies), in order to reduce interference. The advertising device sends a packet on at least one of these three channels, with a repetition period called the advertising interval. For reducing the chance of multiple consecutive collisions, a random delay of up to 10 milliseconds is added to each advertising interval. The scanner listens to the channel for a duration called the scan window, which is periodically repeated every scan interval.
The discovery latency is therefore determined by a probabilistic process and depends on the three parameters (viz., the advertising interval, the scan interval and the scan window). The discovery scheme of BLE adopts a periodic-interval based technique, for which upper bounds on the discovery latency can be inferred for most parametrizations. While the discovery latencies of BLE can be approximated by models67 for purely periodic interval-based protocols, the random delay added to each advertising interval and the three-channel discovery can cause deviations from these predictions, or potentially lead to unbounded latencies for certain parametrizations.68
Security
Bluetooth Low Energy has security instances such as the Encrypted Advertising Data (EAD) feature allowing some or all of the application data payload that is transmitted in advertising packets, to be encrypted. A standard mechanism for the sharing of key material between a broadcasting device and the observers that are intended to receive this data is also defined, so that the data may be decrypted when received.69
All transmitted Bluetooth LE PDUs include a cyclic redundancy check (CRC) that is recalculated and checked by the receiving device for the possibility of the PDU having been changed in flight.69
Software model
All Bluetooth Low Energy devices use the Generic Attribute Profile (GATT). The application programming interface offered by a Bluetooth Low Energy aware operating system will typically be based around GATT concepts.70 GATT has the following terminology:
Client
A device that initiates GATT commands and requests, and accepts responses, for example, a computer or smartphone.
Server
A device that receives GATT commands and requests, and returns responses, for example, a temperature sensor.
Characteristic
A data value transferred between client and server, for example, the current battery voltage.
Service
A collection of related characteristics, which operate together to perform a particular function. For instance, the Health Thermometer service includes characteristics for a temperature measurement value, and a time interval between measurements.
Descriptor
A descriptor provides additional information about a characteristic. For instance, a temperature value characteristic may have an indication of its units (e.g. Celsius), and the maximum and minimum values which the sensor can measure. Descriptors are optional – each characteristic can have any number of descriptors.
Some service and characteristic values are used for administrative purposes – for instance, the model name and serial number can be read as standard characteristics within the Generic Access service. Services may also include other services as sub-functions; the main functions of the device are so-called primary services, and the auxiliary functions they refer to are secondary services.
Identifiers
Services, characteristics, and descriptors are collectively referred to as attributes, and identified by universally unique identifiers (UUIDs). Any implementer may pick a random or pseudorandom UUID for proprietary uses, but the Bluetooth SIG have reserved a range of UUIDs (of the form xxxxxxxx-0000-1000-8000-00805F9B34FB71) for standard attributes. For efficiency, these identifiers are represented as 16-bit or 32-bit values in the protocol, rather than the 128 bits required for a full UUID. For example, the Device Information service has the short code 0x180A, rather than 0000180A-0000-1000-... . The full list is kept in the Bluetooth Assigned Numbers document online.
GATT operations
The GATT protocol provides a number of commands for the client to discover information about the server. These include:
Discover UUIDs for all primary services
Find a service with a given UUID
Find secondary services for a given primary service
Discover all characteristics for a given service
Find characteristics matching a given UUID
Read all descriptors for a particular characteristic
Commands are also provided to read (data transfer from server to client) and write (from client to server) the values of characteristics:
A value may be read either by specifying the characteristic's UUID, or by a handle value (which is returned by the information discovery commands above).
Write operations always identify the characteristic by handle, but have a choice of whether or not a response from the server is required.
'Long read' and 'Long write' operations can be used when the length of the characteristic's data exceeds the MTU of the radio link.
Finally, GATT offers notifications and indications. The client may request a notification for a particular characteristic from the server. The server can then send the value to the client whenever it becomes available. For instance, a temperature sensor server may notify its client every time it takes a measurement. This avoids the need for the client to poll the server, which would require the server's radio circuitry to be constantly operational.
An indication is similar to a notification, except that it requires a response from the client, as confirmation that it has received the message.
Battery impact
Bluetooth Low Energy chipset power consumption when acting as a beacon, as per The Hitchhikers Guide to iBeacon Hardware by Aislelabs.72
Bluetooth Low Energy is designed to enable devices to have very low power consumption. Several chipmakers including Cambridge Silicon Radio, Dialog Semiconductor, Nordic Semiconductor, STMicroelectronics, Cypress Semiconductor, Silicon Labs and Texas Instruments had introduced Bluetooth Low Energy optimized chipsets by 2014. Devices with peripheral and central roles have different power requirements. A study by beacon software company Aislelabs reported that peripherals such as proximity beacons usually function for 1–2 years powered by a 1,000 mAh coin cell battery.73 This is possible because of the power efficiency of Bluetooth Low Energy protocol, which only transmits small packets as compared to Bluetooth Classic which is also suitable for audio and high bandwidth data.
In contrast, a continuous scan for the same beacons in central role can consume 1,000 mAh in a few hours. Android and iOS devices also have very different battery impact depending on type of scans and the number of Bluetooth Low Energy devices in the vicinity.74 With newer chipsets and advances in software, by 2014 both Android and iOS phones had negligible power consumption in real-life Bluetooth Low Energy use.75
2M PHY
Bluetooth 5 has introduced a new transmission mode with a doubled symbol rate. Bluetooth LE has been traditionally transmitting 1 bit per symbol so that theoretically the data rate doubles as well. However, the new mode doubles the bandwidth from about 1 MHz to about 2 MHz which makes for more interferences on the edge regions. The partitioning of the ISM frequency band has not changed being still 40 channels spaced at a distance of 2 MHz.76 This is an essential difference over Bluetooth 2 EDR which also doubled the data rate but was doing that by employing a π/4-DQPSK or 8-DPSK phase modulation on a 1 MHz channel while Bluetooth 5 continues to use just frequency shift keying.
The traditional transmission of 1 Mbit in the Bluetooth Basic Rate was renamed 1M PHY in Bluetooth 5. The new mode at a doubled symbol speed was introduced as the 2M PHY. In Bluetooth Low Energy every transmission starts on the 1M PHY leaving it to the application to initiate a switch to the 2M PHY. In that case both sender and receiver will switch to the 2M PHY for transmissions. This is designed to facilitate firmware updates where the application can switch back to a traditional 1M PHY in case of errors. In reality the target device should be close to the programming station (at a few meters).
High Data Throughput
Bluetooth 7 introduced a new transmission mode, High Data Throughput (HDT). It nearly quadruples the maximum throughput from 2 Mbit to 7.5 Mbit. This increased data rate improves Bluetooth's suitability for high bandwidth applications, such as lossless audio, AI, and extended reality headsets.7778 Bluetooth HDT increases the symbol rate and changes the modulation scheme to 16QAM at the highest throughput mode.79
Bluetooth 7 devices with HDT are only required to support HDT2, HDT3, and HDT4, with the latter supporting 4 Mbit throughput. HDT6 and HDT7.5 is optional.7977
LE Coded
Bluetooth 5 has introduced two new modes with lower data rates. The symbol rate of the new "Coded PHY" is the same as the Base Rate 1M PHY, but in mode S=2 there are two symbols transmitted per data bit. In mode S=2 only a simple Pattern Mapping P=1 is used which produces the same stuffing bit for each input data bit. In mode S=8 there are eight symbols per data bit, with a Pattern Mapping P=4, producing contrasting symbol sequences – a 0 bit is encoded as binary 0011 and a 1 bit is encoded as binary 1100.80 In mode S=2 using P=1 the range roughly doubles, while in mode S=8 using P=4 it roughly quadruples.81
Not only have the "LE Coded" transmissions changed the error correction scheme, but they use a fundamentally new packet format. Each "LE Coded" burst consists of three blocks. The switch block ("extended preamble") is transmitted on the LE 1M PHY but it only consists of the binary pattern '00111100' repeated 10 times. These 80 bits are not FEC encoded as usual, but are sent directly to the radio channel. It is followed by a header block ("FEC Block 1") which is always transmitted in S=8 mode. The header block only contains the destination address ("Access Address" / 32 bit) and an encoding flag ("Coding Indicator" / 2 Bit). The Coding Indicator defines the Pattern Mapping used for the following payload block ("FEC Block 2") where it is possible to use S=2 mode.82
The new packet format of Bluetooth 5 allows transmission of 2 to 256 bytes as the payload in a single burst. This is a lot more than the maximum of 31 bytes in Bluetooth 4. Along with reach measurements this should allow for localization functions. As a whole, the quadrupled range—at the same transmission power—is achieved at the expense of a lower data rate, being 125 kbit/s, one eighth of the usual. The old transmission packet format, as it continues to be used in the 1M PHY and 2M PHY modes, has been named "Uncoded" in Bluetooth 5. The intermediate "LE Coded" S=2 mode allows for a 500 kbit/s data rate in the payload which is both beneficial for shorter latencies as well as lower power consumption, as the burst time itself is shorter.
12Honkanen, M.; Lappetelainen, A.; Kivekas, K. (2004). Low end extension for Bluetooth. 2004 IEEE Radio and Wireless Conference, 19–22 September 2004. IEEE. pp. 199–202. doi:10.1109/RAWCON.2004.1389107.
17Reynolds, Melanie (12 June 2007). "Wibree becomes ULP Bluetooth". ElectronicsWeekly.com. Reed Business Information Limited. Archived from the original on 7 September 2008. Retrieved 9 September 2008.
61Padovan, Gustavo (22 February 2013). "The big changes of BlueZ 5". Archived from the original on 23 September 2017. Retrieved 2 June 2014. “As the MGMT interface is the only one to support the new Bluetooth Low Energy devices, BlueZ developers decided to drop support for the old interface once MGMT was completed. As a result, you need to be running Linux Kernel 3.4 or newer to use BlueZ 5.”
Temporary account§ LE CodedThis needed attention from a native speaker of English. I'm sure the original translator did their best, and I'm grateful for their effort. I hope I've made the wording less awkward.+37diff
Network of devices used to wirelessly locate objects inside a building
An indoor positioning system (IPS) is a network of devices used to locate people or objects where GPS and other satellite technologies lack precision or fail entirely, such as inside multistory buildings, airports, alleys, parking garages, and underground locations.
Physical fitness is a state of health and well-being and, more specifically, the ability to perform aspects of sports, occupations, and daily activities.
Family of Unix-like operating systems that use the Linux kernel
Linux (LIN-uuks) is a family of free and open-source software Unix-like operating systems based on the Linux kernel, which was first released on 17 September 1991 by Linus Torvalds.
Open-source wireless sensor and actuator network protocol
DASH7 Alliance Protocol (D7A) is an open-source wireless sensor and actuator network protocol, which operates in the 433 MHz, 868 MHz and 915 MHz unlicensed ISM/SRD band.
Bluetooth beacons are hardware transmitters (a class of Bluetooth Low Energy (LE) devices) that broadcast their identifier to nearby portable electronic devices.
macOS (previously OS X and originally Mac OS X) is a proprietary Unix operating system, derived from OPENSTEP for Mach and FreeBSD, which has been marketed and developed by Apple since 2001.
BlackBerry 10 (BB10) is a deprecated proprietary mobile operating system for the BlackBerry line of smartphones, developed by BlackBerry Limited (formerly known as Research In Motion).
Very low energy, short range radio precise positioning
Ultra-wideband (UWB, ultra wideband, ultra-wide band and ultraband) is a radio technology that can use a very low energy level for short-range, high-bandwidth communications over a large portion of the radio spectrum.
Electromagnetic frequencies ranging from 9 kHz to 300 GHz
Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around 9 kHz to around 300 GHz.
The Bluetooth Low Energy denial of service attacks are a series of denial-of-service attacks against mobile phones and iPads via Bluetooth Low Energy that can make it difficult to use them.
In radio-frequency engineering, an antenna (American English) or aerial (British English) is a structure used to convert alternating electric currents into radio waves for transmission, and to convert radio waves back into electric currents for reception.
A button cell, watch battery, or coin battery is a small battery made of a single electrochemical cell and shaped as a squat cylinder typically 5 to 25 mm (0.197 to 0.984 in) in diameter and 1 to 6 mm (0.039 to 0.236 in) high – resembling a button.
