Bit

2025-07-08T22:24:43Z

2.196.217.134: removed an example (not indicated in the linked article considering redirect)

{{short description|Unit of information}}
{{About|the unit of information}}
{{Use dmy dates|date=December 2020|cs1-dates=y}}
{{Fundamental info units}}

The '''bit''' is the most basic [[Units of information|unit of information]] in [[computing]] and digital [[communication]]. The name is a [[portmanteau]] of '''binary digit'''.<ref name="Mackenzie_1980"/> The bit represents a [[truth value|logical state]] with one of two possible [[value (computer science)|values]]. These values are most commonly represented as either {{nowrap|"{{mono|1}}" or "{{mono|0}}"}}, but other representations such as ''true''/''false'', ''yes''/''no'', ''on''/''off'', or ''+''/''−'' are also widely used.

The relation between these values and the physical states of the underlying [[Data storage device|storage]] or [[computing device|device]] is a matter of convention, and different assignments may be used even within the same device or [[computer program|program]]. It may be physically implemented with a two-state device.

A contiguous group of binary digits is commonly called a ''[[bit string]]'', a bit vector, or a single-dimensional (or multi-dimensional) ''[[bit array]]''. A group of eight bits is called one ''[[byte]]'', but historically the size of the byte is not strictly defined.<ref name="Bemer_2000"/> Frequently, half, full, double and quadruple [[Word (computer architecture)|words]] consist of a number of bytes which is a low power of two. A string of four bits is usually a ''[[nibble]]''.

In [[information theory]], one bit is the [[information entropy]] of a random [[Binary number|binary]] variable that is 0 or 1 with equal probability,<ref name="Anderson_2006"/> or the information that is gained when the value of such a variable becomes known.<ref name="Haykin_2006"/><ref name="IEEE_260"/> As a [[unit of information]], the bit is also known as a ''[[shannon (unit)|shannon]]'',<ref name="Rowlett"/> named after [[Claude E. Shannon]]. As a measure of the length of a digital string that is encoded as symbols over a 0-1 (binary) alphabet, the bit has been called a binit,<ref>{{cite book |last1=Breipohl |first1=Arthur M. |title=Adaptive Communication Systems |date=1963-08-18 |publisher=University of New Mexico |page=7 |url=https://digitalrepository.unm.edu/ece_etds/425/ |access-date=7 January 2025}}</ref> but this usage is now rare.<ref>{{cite dictionary |title=binit |url=https://www.thefreedictionary.com/binit |dictionary=The Free Dictionary |access-date=7 January 2025}}</ref>

In [[data compression]], the goal is to find a shorter representation for a string, so that it requires fewer bits when stored or transmitted; the string would be compressed into the shorter representation before doing so, and then decompressed into its original form when read from storage or received. The field of [[algorithmic information theory]] is devoted to the study of the irreducible information content of a string (i.e., its shortest-possible representation length, in bits), under the assumption that the receiver has minimal ''a priori'' knowledge of the method used to compress the string. In [[error detection and correction]], the goal is to add redundant data to a string, to enable the detection or correction of errors during storage or transmission; the redundant data would be computed before doing so, and stored or transmitted, and then checked or corrected when the data is read or received.

The symbol for the binary digit is either "bit", per the [[IEC 80000-13]]:2008 standard, or the lowercase character "b", per the [[IEEE 1541-2002]] standard. Use of the latter may create confusion with the capital "B" which is the international standard symbol for the byte.

== History ==

[[Ralph Hartley]] suggested the use of a logarithmic measure of information in 1928.<ref name="Abramson_1963"/> [[Claude E. Shannon]] first used the word "bit" in his seminal 1948 paper "[[A Mathematical Theory of Communication]]".<ref name="Shannon_1948_1"/><ref name="Shannon_1948_2"/><ref name="Shannon_1949"/> He attributed its origin to [[John W. Tukey]], who had written a Bell Labs memo on 9 January 1947 in which he contracted "binary information digit" to simply "bit".<ref name="Shannon_1948_1"/>

== Physical representation ==

A bit can be stored by a digital device or other physical system that exists in either of two possible distinct [[state (computer science)|states]]. These may be the two stable states of a [[Flip-flop (electronics)|flip-flop]], two positions of an [[Switch|electrical switch]], two distinct [[voltage]] or [[electric current|current]] levels allowed by a [[electrical circuit|circuit]], two distinct levels of [[Irradiance|light intensity]], two directions of [[magnetism|magnetization]] or [[electrical polarity|polarization]], the orientation of reversible double stranded [[DNA]], etc.

Perhaps the earliest example of a binary storage device was the [[punched card]] invented by [[Basile Bouchon]] and Jean-Baptiste Falcon (1732), developed by [[Joseph Marie Jacquard]] (1804), and later adopted by [[Semyon Korsakov]], [[Charles Babbage]], [[Herman Hollerith]], and early computer manufacturers like [[IBM]]. A variant of that idea was the perforated [[paper tape]]. In all those systems, the medium (card or tape) conceptually carried an array of hole positions; each position could be either punched through or not, thus carrying one bit of information. The encoding of text by bits was also used in [[Morse code]] (1844) and early digital communications machines such as [[Teleprinter|teletypes]] (1870).

The first electrical devices for discrete logic (such as [[elevator]] and [[traffic light]] control [[Electronic circuit|circuits]], [[telephone switches]], and Konrad Zuse's computer) represented bits as the states of [[electrical relay]]s which could be either "open" or "closed". These relays functioned as mechanical switches, physically toggling between states to represent binary data, forming the fundamental building blocks of early computing and control systems. When relays were replaced by [[vacuum tube]]s, starting in the 1940s, computer builders experimented with a variety of storage methods, such as pressure pulses traveling down a [[mercury delay line]], charges stored on the inside surface of a [[cathode-ray tube]], or opaque spots printed on [[optical disc|glass discs]] by [[photolithographic]] techniques.

In the 1950s and 1960s, these methods were largely supplanted by [[magnetic storage]] devices such as [[magnetic-core memory]], [[magnetic tape]]s, [[magnetic drum|drums]], and [[Disk storage|disks]], where a bit was represented by the polarity of [[magnetism|magnetization]] of a certain area of a [[ferromagnetic]] film, or by a change in polarity from one direction to the other. The same principle was later used in the [[magnetic bubble memory]] developed in the 1980s, and is still found in various [[magnetic strip]] items such as [[Rapid transit|metro]] tickets and some [[credit card]]s.

In modern [[semiconductor memory]], such as [[dynamic random-access memory]] or a [[solid-state drive]], the two values of a bit are represented by two levels of [[electric charge]] stored in a [[capacitor]] or a [[floating-gate MOSFET]]. In certain types of [[programmable logic array]]s and [[read-only memory]], a bit may be represented by the presence or absence of a conducting path at a certain point of a circuit. In [[optical disc]]s, a bit is encoded as the presence or absence of a [[microscopic]] pit on a reflective surface. In one-dimensional [[bar code]]s and two-dimensional [[QR codes]], bits are encoded as lines or squares which may be either black or white.

In modern digital computing, bits are transformed in Boolean [[logic gate]]s.

=== Transmission and processing ===
Bits are transmitted one at a time in [[serial transmission]]. By contrast, multiple bits are transmitted simultaneously in a [[parallel transmission]]. A [[serial computer]] processes information in either a bit-serial or a byte-serial fashion. From the standpoint of data communications, a byte-serial transmission is an 8-way parallel transmission with binary signalling.

In programming languages such as [[C (programming language)|C]], a [[bitwise operation]] operates on binary strings as though they are vectors of bits, rather than interpreting them as [[binary number]]s.

Data transfer rates are usually measured in decimal SI multiples. For example, a [[channel capacity]] may be specified as 8 kbit/s = 1 kB/s.

=== Storage ===

File sizes are often measured in (binary) IEC multiples of bytes, for example 1 KiB = 1024 bytes = 8192 bits. Confusion may arise in cases where (for historic reasons) filesizes are specified with binary multipliers using the ambiguous prefixes K, M, and G rather than the IEC standard prefixes Ki, Mi, and Gi.<ref>{{cite web |title=UnitsPolicy - Ubuntu Wiki |url=https://wiki.ubuntu.com/UnitsPolicy |access-date=7 January 2025}}</ref>

Mass storage devices are usually measured in decimal SI multiples, for example 1 TB = <math>10^{12}</math> bytes.

Confusingly, the storage capacity of a directly addressable memory device, such as a [[Dynamic Random Access Memory|DRAM]] chip, or an assemblage of such chips on a memory module, is specified as a binary multiple—using the ambiguous prefix G rather than the IEC recommended Gi prefix. For example, a DRAM chip that is specified (and advertised) as having "1 GB" of capacity has <math>2^{30}</math> bytes of capacity. As at 2022, the difference between the popular understanding of a memory system with "8 GB" of capacity, and the SI-correct meaning of "8 GB" was still causing difficulty to software designers.<ref>{{cite web |title=Use MB/GB/TB suffix for VM memory input |url=https://github.com/netbox-community/netbox/issues/8437 |website=Github Netbox Community |access-date=8 January 2025 |date=2022}}</ref>

== Unit and symbol ==
The bit is not defined in the [[International System of Units]] (SI). However, the [[International Electrotechnical Commission]] issued standard [[IEC 60027]], which specifies that the symbol for binary digit should be 'bit', and this should be used in all multiples, such as 'kbit', for kilobit.<ref name="NIST_2008"/> However, the lower-case letter 'b' is widely used as well and was recommended by the [[IEEE 1541-2002|IEEE 1541 Standard (2002)]]. In contrast, the upper case letter 'B' is the standard and customary symbol for byte.

=== Multiple bits ===
{{redirect2|MBit|Tbit|the technical high school|MBIT|the international terminal in Los Angeles International Airport (LAX)|TBIT}}
{{Quantities of bits}}
Multiple bits may be expressed and represented in several ways. For convenience of representing commonly reoccurring groups of bits in information technology, several [[units of information]] have traditionally been used. The most common is the unit [[byte]], coined by [[Werner Buchholz]] in June 1956, which historically was used to represent the group of bits used to encode a single [[character (computing)|character]] of text (until [[UTF-8]] multibyte encoding took over) in a computer<ref name="Bemer_2000"/><ref name="Buchholz_1956"/><ref name="Buchholz_1977"/><ref name="Buchholz_1962"/><ref name="Bemer_1959"/> and for this reason it was used as the basic [[address space|addressable]] element in many [[computer architecture]]s. By 1993, the trend in hardware design had converged on the 8-bit [[byte]].<ref>{{cite web |title=ISO/IEC 2382-1:1993(en) Information technology — Vocabulary — Part 1: Fundamental terms |url=https://www.iso.org/obp/ui/#iso:std:iso-iec:2382:-1:ed-3:v1:en |access-date=8 January 2025 |page=01.02.09}}</ref> However, because of the ambiguity of relying on the underlying hardware design, the unit [[Octet (computing)|octet]] was defined to explicitly denote a sequence of eight bits.

Computers usually manipulate bits in groups of a fixed size, conventionally named "[[Word (computer architecture)|words]]". Like the byte, the number of bits in a word also varies with the hardware design, and is typically between 8 and 80 bits, or even more in some specialized computers. In the early 21st century, retail personal or server computers have a word size of 32 or 64 bits.

The [[International System of Units]] defines a series of decimal prefixes for multiples of standardized units which are commonly also used with the bit and the byte. The prefixes [[kilo-|kilo]] (103) through [[yotta-|yotta]] (1024) increment by multiples of one thousand, and the corresponding units are the [[kilobit]] (kbit) through the [[yottabit]] (Ybit).

== See also ==
* {{Annotated link|Baud}}
* {{Annotated link|Binary numeral system}}
* {{Annotated link|Bit rate}}
* {{Annotated link|Bitstream}}
* {{Annotated link|Entropy (information theory)}}
* {{Annotated link|Fuzzy bit}}
* {{Annotated link|Integer (computer science)}}
* {{Annotated link|Primitive data type}}
* {{Annotated link|Qubit}} (quantum bit)
* {{Annotated link|Shannon (unit)}}
* Trit – {{Annotated link|Ternary numeral system}} (ternary digit)

== References ==
{{reflist|refs=
<ref name="Mackenzie_1980">{{cite book |url=https://textfiles.meulie.net/bitsaved/Books/Mackenzie_CodedCharSets.pdf |title=Coded Character Sets, History and Development |series=The Systems Programming Series |author-last=Mackenzie |author-first=Charles E. |date=1980 |edition=1 |publisher=[[Addison-Wesley Publishing Company, Inc.]] |isbn=978-0-201-14460-4 |lccn=77-90165 |page=x |access-date=2019-08-25 |archive-url=https://web.archive.org/web/20160526172151/https://textfiles.meulie.net/bitsaved/Books/Mackenzie_CodedCharSets.pdf |archive-date=May 26, 2016 |url-status=live |df=mdy-all }}</ref>
<ref name="Anderson_2006">{{citation |author-first1=John B. |author-last1=Anderson |author-first2=Rolf |author-last2=Johnnesson |date=2006 |title=Understanding Information Transmission}}</ref>
<ref name="Haykin_2006">{{citation |author-first=Simon |author-last=Haykin |date=2006 |title=Digital Communications}}</ref>
<ref name="IEEE_260">[[IEEE Std 260.1-2004]]</ref>
<ref name="Rowlett">{{cite web |url=https://www.unc.edu/~rowlett/units/dictB.html#bit |title=Units: B |url-status=live |archive-url=https://web.archive.org/web/20160504055432/http://www.unc.edu/~rowlett/units/dictB.html#bit |archive-date=2016-05-04}}</ref>
<ref name="Abramson_1963">{{cite book |author-first=Norman |author-last=Abramson |date=1963 |title=Information theory and coding |publisher=[[McGraw-Hill]]}}</ref>
<ref name="NIST_2008">National Institute of Standards and Technology (2008), ''Guide for the Use of the International System of Units''. [http://physics.nist.gov/cuu/pdf/sp811.pdf Online version.] {{webarchive|url=https://web.archive.org/web/20160603203340/http://physics.nist.gov/cuu/pdf/sp811.pdf |date=3 June 2016}}</ref>

<ref name="Shannon_1948_1">{{cite journal |author-last=Shannon |author-first=Claude Elwood |author-link=Claude Elwood Shannon |title=A Mathematical Theory of Communication |journal=[[Bell System Technical Journal]] |volume=27 |issue=3 |pages=379–423 |date=July 1948 |doi=10.1002/j.1538-7305.1948.tb01338.x |hdl=11858/00-001M-0000-002C-4314-2 |url=http://cm.bell-labs.com/cm/ms/what/shannonday/shannon1948.pdf |via=Bell Labs Computing and Mathematical Sciences Research |archive-url=https://web.archive.org/web/19980715013250/http://cm.bell-labs.com/cm/ms/what/shannonday/shannon1948.pdf |url-status=dead |archive-date=1998-07-15 |quote=The choice of a logarithmic base corresponds to the choice of a unit for measuring information. If the base 2 is used the resulting units may be called binary digits, or more briefly ''bits'', a word suggested by [[John Wilder Tukey|J. W. Tukey]].|hdl-access=free }}</ref>
<ref name="Shannon_1948_2">{{cite journal |author-last=Shannon |author-first=Claude Elwood |author-link=Claude Elwood Shannon |title=A Mathematical Theory of Communication |journal=[[Bell System Technical Journal]] |volume=27 |issue=4 |pages=623–666 |date=October 1948 |doi=10.1002/j.1538-7305.1948.tb00917.x |hdl=11858/00-001M-0000-002C-4314-2|hdl-access=free }}</ref>
<ref name="Shannon_1949">{{cite book |author-last1=Shannon |author-first1=Claude Elwood |author-link1=Claude Elwood Shannon |author-first2=Warren |author-last2=Weaver |author-link2=Warren Weaver |title=A Mathematical Theory of Communication |publisher=[[University of Illinois Press]] |date=1949 |isbn=0-252-72548-4 |url=http://cm.bell-labs.com/cm/ms/what/shannonday/shannon1948.pdf |via=Bell Labs Computing and Mathematical Sciences Research |archive-url=https://web.archive.org/web/19980715013250/http://cm.bell-labs.com/cm/ms/what/shannonday/shannon1948.pdf |url-status=dead |archive-date=1998-07-15}}</ref>
<ref name="Bemer_2000">{{cite web |title=Why is a byte 8 bits? Or is it? |author-first=Robert William |author-last=Bemer |author-link=Robert William Bemer |date=2000-08-08 |work=Computer History Vignettes |url=http://www.bobbemer.com/BYTE.HTM |access-date=2017-04-03 |url-status=dead |archive-url=https://web.archive.org/web/20170403130829/http://www.bobbemer.com/BYTE.HTM |archive-date=2017-04-03 |quote=[...] With [[IBM]]'s [[IBM STRETCH|STRETCH]] computer as background, handling 64-character words divisible into groups of 8 (I designed the character set for it, under the guidance of Dr. [[Werner Buchholz]], the man who DID coin the term "[[byte]]" for an 8-bit grouping). [...] The [[IBM System 360|IBM 360]] used 8-bit characters, although not ASCII directly. Thus Buchholz's "byte" caught on everywhere. I myself did not like the name for many reasons. [...]}}</ref>
<ref name="Buchholz_1956">{{cite book |title=The Link System |chapter=7. The Shift Matrix |author-first=Werner |author-last=Buchholz |author-link=Werner Buchholz |date=1956-06-11 |id=[[IBM Stretch|Stretch]] Memo No. 39G |publisher=[[IBM]] |pages=5–6 |chapter-url=http://archive.computerhistory.org/resources/text/IBM/Stretch/pdfs/06-07/102632284.pdf |access-date=2016-04-04 |url-status=live |archive-url=https://web.archive.org/web/20170404152534/http://archive.computerhistory.org/resources/text/IBM/Stretch/pdfs/06-07/102632284.pdf |archive-date=2017-04-04 |quote=[...] Most important, from the point of view of editing, will be the ability to handle any characters or digits, from 1 to 6 bits long [...] the Shift Matrix to be used to convert a 60-bit [[word (computer architecture)|word]], coming from Memory in parallel, into [[character (computing)|characters]], or "[[byte]]s" as we have called them, to be sent to the [[serial adder|Adder]] serially. The 60 bits are dumped into [[magnetic core]]s on six different levels. Thus, if a 1 comes out of position 9, it appears in all six cores underneath. [...] The Adder may accept all or only some of the bits. [...] Assume that it is desired to operate on 4 bit [[decimal digit]]s, starting at the right. The 0-diagonal is pulsed first, sending out the six bits 0 to 5, of which the Adder accepts only the first four (0-3). Bits 4 and 5 are ignored. Next, the 4 diagonal is pulsed. This sends out bits 4 to 9, of which the last two are again ignored, and so on. [...] It is just as easy to use all six bits in [[alphanumeric]] work, or to handle bytes of only one bit for logical analysis, or to offset the bytes by any number of bits. [...]}}</ref>
<ref name="Buchholz_1977">{{cite journal |author-last=Buchholz |author-first=Werner |author-link=Werner Buchholz |title=The Word "Byte" Comes of Age... |journal=[[Byte Magazine]] |date=February 1977 |volume=2 |issue=2 |page=144 |url=https://archive.org/stream/byte-magazine-1977-02/1977_02_BYTE_02-02_Usable_Systems#page/n145/mode/2up |quote=[...] The first reference found in the files was contained in an internal memo written in June 1956 during the early days of developing [[IBM Stretch|Stretch]]. A [[byte]] was described as consisting of any number of parallel bits from one to six. Thus a byte was assumed to have a length appropriate for the occasion. Its first use was in the context of the input-output equipment of the 1950s, which handled six bits at a time. The possibility of going to 8 bit bytes was considered in August 1956 and incorporated in the design of Stretch shortly thereafter. The first published reference to the term occurred in 1959 in a paper "Processing Data in Bits and Pieces" by [[Gerrit Anne Blaauw|G A Blaauw]], [[Frederick Phillips Brooks, Jr.|F P Brooks Jr]] and [[Werner Buchholz|W Buchholz]] in the ''[[IRE Transactions on Electronic Computers]]'', June 1959, page 121. The notions of that paper were elaborated in Chapter 4 of ''[[#Buchholz-1962|Planning a Computer System (Project Stretch)]]'', edited by W Buchholz, [[McGraw-Hill Book Company]] (1962). The rationale for coining the term was explained there on page 40 as follows: Byte ''denotes a group of bits used to encode a character, or the number of bits transmitted in parallel to and from input-output units. A term other than ''character'' is used here because a given character may be represented in different applications by more than one code, and different codes may use different numbers of bits (ie, different byte sizes). In input-output transmission the grouping of bits may be completely arbitrary and have no relation to actual characters. (The term is coined from ''[[bite]]'', but respelled to avoid accidental mutation to ''bit''.)'' [[System/360]] took over many of the Stretch concepts, including the basic byte and word sizes, which are powers of 2. For economy, however, the byte size was fixed at the 8 bit maximum, and addressing at the bit level was replaced by byte addressing. [...]}}</ref>
<ref name="Buchholz_1962">{{anchor|Buchholz-1962}}{{citation |title=Planning a Computer System – Project Stretch |author-first1=Gerrit Anne |author-last1=Blaauw |author-link1=Gerrit Anne Blaauw |author-first2=Frederick Phillips |author-last2=Brooks, Jr. |author-link2=Frederick Phillips Brooks, Jr. |author-first3=Werner |author-last3=Buchholz |author-link3=Werner Buchholz |editor-first=Werner |editor-last=Buchholz |editor-link=Werner Buchholz |publisher=[[McGraw-Hill Book Company, Inc.]] / The Maple Press Company, York, PA. |lccn=61-10466 |date=1962 |chapter=Chapter 4: Natural Data Units |pages=39–40 |chapter-url=http://archive.computerhistory.org/resources/text/IBM/Stretch/pdfs/Buchholz_102636426.pdf |access-date=2017-04-03 |url-status=dead |archive-url=https://web.archive.org/web/20170403014651/http://archive.computerhistory.org/resources/text/IBM/Stretch/pdfs/Buchholz_102636426.pdf |archive-date=2017-04-03}}</ref>
<ref name="Bemer_1959">{{cite journal |author-first=Robert William |author-last=Bemer |author-link=Robert William Bemer |title=A proposal for a generalized card code of 256 characters |journal=[[Communications of the ACM]] |volume=2 |number=9 |pages=19–23 |date=1959 |doi=10.1145/368424.368435|s2cid=36115735 |doi-access=free }}</ref>
}}

== External links ==
{{wiktionary}}
* [https://web.archive.org/web/20090216151053/http://www.bit-calculator.com/ Bit Calculator] – a tool providing conversions between bit, byte, kilobit, kilobyte, megabit, megabyte, gigabit, gigabyte
* [http://nxu.biz/tools/BitXByteConverter/ BitXByteConverter] {{Webarchive|url=https://web.archive.org/web/20160406223558/http://nxu.biz/tools/BitXByteConverter/ |date=2016-04-06 }} – a tool for computing file sizes, storage capacity, and digital information in various units

{{Information units}}
{{Data types}}
{{Authority control}}

[[Category:Binary arithmetic]]
[[Category:Primitive types]]
[[Category:Data types]]
[[Category:Units of information]]

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