# Prefix code > Mediated Wiki article. Canonical URL: https://mediated.wiki/source/Prefix_code > Markdown URL: https://mediated.wiki/source/Prefix_code.md > Source: https://en.wikipedia.org/wiki/Prefix_code > Source revision: 1347243775 > License: Creative Commons Attribution-ShareAlike 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) Set of codewords, none a prefix of another A **prefix code** is a type of [code](/source/Code) system distinguished by its possession of the **prefix property**, which requires that there is no whole [code word](/source/Code_word_(communication)) in the system that is a [prefix](/source/Prefix_(computer_science)) (initial segment) of any other code word in the system. It is trivially true for fixed-length codes, so only a point of consideration for [variable-length codes](/source/Variable-length_code). For example, a code with code { a , bb } {\displaystyle \{{\texttt {a}},{\texttt {bb}}\}} has the prefix property; a code consisting of { a , b , ab , aa } {\displaystyle \{{\texttt {a}},{\texttt {b}},{\texttt {ab}},{\texttt {aa}}\}} does not, because a is a prefix of ab and also of aa. A prefix code is a [uniquely decodable code](/source/Uniquely_decodable_code): given a complete and accurate sequence, a receiver can identify each word without requiring a special marker between words. However, there are uniquely decodable codes that are not prefix codes; for instance, the reverse of a prefix code is still uniquely decodable (it is a suffix code), but it is not necessarily a prefix code. Prefix codes are also known as **prefix-free codes**, **prefix condition codes** and **instantaneous codes**. Although [Huffman coding](/source/Huffman_coding) is just one of many algorithms for deriving prefix codes, prefix codes are also widely referred to as "Huffman codes", even when the code was not produced by a Huffman algorithm. The term **comma-free code** is sometimes also applied as a synonym for prefix-free codes[1][2] but in most mathematical books and articles (e.g.[3][4]) a comma-free code is used to mean a [self-synchronizing code](/source/Self-synchronizing_code), a subclass of prefix codes. Using prefix codes, a message can be transmitted as a sequence of concatenated code words, without any [out-of-band](/source/Out-of-band_data) markers or (alternatively) special markers between words to [frame](/source/Framing_(telecommunication)) the words in the message. The recipient can decode the message unambiguously, by repeatedly finding and removing sequences that form valid code words. This is not generally possible with codes that lack the prefix property, for example { 0 , 1 , 10 , 11 } {\displaystyle \{{\texttt {0}},{\texttt {1}},{\texttt {10}},{\texttt {11}}\}} : a receiver reading a 1 at the start of a code word would not know whether that was the complete code word 1, or merely the prefix of the code word 10 or 11; so the string 10 could be interpreted either as a single codeword or as the concatenation of the words 1 then 0. The variable-length [Huffman codes](/source/Huffman_coding), [telephone country codes](/source/Telephone_country_code), the country and publisher parts of [ISBNs](/source/ISBN), the Secondary Synchronization Codes used in the [UMTS](/source/UMTS) [W-CDMA](/source/W-CDMA) 3G Wireless Standard, and the [instruction sets](/source/Instruction_set) (machine language) of most computer microarchitectures are prefix codes. Prefix codes are not [error-correcting codes](/source/Error-correcting_codes). In practice, a message might first be compressed with a prefix code, and then encoded again with [channel coding](/source/Channel_coding) (including error correction) before transmission. For every [uniquely decodable](/source/Variable-length_code#Uniquely_decodable_codes) code there is a prefix code that has the same code word lengths.[5] [Kraft's inequality](/source/Kraft's_inequality) characterizes the sets of code word lengths that are possible in a uniquely decodable code.[6] ## Techniques If every word in the code has the same length, the code is called a **fixed-length code**, or a **block code** (though the term [block code](/source/Block_code) is also used for fixed-size [error-correcting codes](/source/Error-correcting_code) in [channel coding](/source/Channel_coding)). For example, [ISO 8859-15](/source/ISO_8859-15) letters are always 8 bits long. [UTF-32/UCS-4](/source/UTF-32%2FUCS-4) letters are always 32 bits long. [ATM cells](/source/Asynchronous_Transfer_Mode) are always 424 bits (53 bytes) long. A fixed-length code of fixed length k {\displaystyle k} bits can encode up to 2 k {\displaystyle 2^{k}} source symbols. A fixed-length code is necessarily a prefix code. It is possible to turn any code into a fixed-length code by padding fixed symbols to the shorter prefixes in order to meet the length of the longest prefixes. Alternately, such padding codes may be employed to introduce redundancy that allows autocorrection and/or synchronisation. However, fixed length encodings are inefficient in situations where some words are much more likely to be transmitted than others. [Truncated binary encoding](/source/Truncated_binary_encoding) is a straightforward generalization of fixed-length codes to deal with cases where the number of symbols *n* is not a power of two. Source symbols are assigned codewords of length k {\displaystyle k} and k + 1 {\displaystyle k+1} , where k {\displaystyle k} is chosen so that 2 k < n ≤ 2 k + 1 {\displaystyle 2^{k}