Alphanumericals
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Alphanumeric (sometimes shortened to alphameric) is a combination of alphabetic and numeric characters, and is used to describe the collection of Latin letters and Arabic digits or a text constructed from this collection. There are either 36 (single case) or 62 (case-sensitive) alphanumeric characters. The alphanumeric character set consists of the numbers 0 to 9 and letters A to Z.
Computing
In computing terminology, a character stored in alphanumeric form may potentially occupy less space than storing an 8-bit ASCII character, as each character requires only 6 bits for encoding.
There is no standard for storing 6-bit alphanumeric data. A 6-bit field has 64 possible values, so if only 36 are used in single case, there is room for another 28 characters (usually the lowercase alphabet and other punctuation), making alphanumeric data useful for storing text and website addresses.
Subsets of alphanumeric used in human interfaces
When a string of mixed alphabets and numerals is presented for human interpretation, ambiguities arise. The most obvious is the similarity of the letters I, O and Q to the numbers 1 and 0. Therefore, depending on the application, various subsets of the alphanumeric were adopted to avoid misinterpretation by humans.
In passenger aircraft, aircraft seat maps and seats were designated by row number followed by column letter. For wide bodied jets, the seats can be 10 across, labeled ABC-DEFG-HJK. The letter I is skipped to avoid mistaking it as row number 1.
In Vehicle Identification Number used by motor vehicle manufacturers, the letters I, O and Q are omitted for their similarity to 1 or 0.
Tiny embossed letters are used to label pins on an V.35/M34 electrical connector. The letters I, O, Q, S and Z were dropped to ease eye strain with 1, 0, 5, and 2. That subset is named DEC Alphabet after the company that first used it.
For alphanumerics that are frequently handwritten, in addition to I and O, V is avoided because it looks like U in cursive, and Z for its similarity to 2.
Other references
In the TV show ReBoot, "alphanumeric" was an interjection used by Enzo as an equivalent of "cool!" or "awesome!" In the episode "Talent Night", it was used to mean "everything is all right."