X-bar theory used a bar over syntactic units to indicate bar-levels in syntactic structure, generally rendered as an overbar. While easy to write, the bar notation proved difficult to typeset, leading to the adoption of the prime symbol to indicate a bar. (Despite the lack of bar, the unit would still be read as "X bar", as opposed to "X prime".) With contemporary development of typesetting software such as LaTeX, typesetting bars is considerably simpler; nevertheless, both prime and bar markups are accepted usages. Some X-bar notations use a double-prime (standing in for a double-bar) to indicate a phrasal level, indicated in most notations by "XP". Use in music The prime symbol is used in combination with lower case letters in the Helmholtz pitch notation system to distinguish notes in different octaves from middle C upwards. Thus represents the below middle C, represents middle C, represents the in the octave above middle C, and the in the octave two octaves above middle C. A combination of upper case letters and sub-prime symbols is used to represent notes in lower octaves. Thus represents the below the bass stave, while represents the in the octave below that. In some musical scores, the double prime is used to indicate a length of time in seconds. It is used over a fermata denoting a long note or rest. Computer encodings Unicode and HTML representations of the prime and related symbols are as follows. (lower case p) (upper case P) The "modifier letter prime" and "modifier letter double prime" characters are intended for linguistic purposes, such as the indication of stress or the transliteration of certain Cyrillic characters. In a context when the character set used does not include the prime or double prime character (e.g., in an online discussion context where only ASCII or ISO 8859-1 [ISO Latin 1] is expected), they are often respectively approximated by ASCII apostrophe (U+0027) or quotation mark (U+0022). LaTeX provides an oversized prime symbol, (), which, when used in super- or sub-scripts, renders appropriately; e.g., f_\prime^\prime appears as . An apostrophe, , is a shortcut for a superscript prime; e.g., appears as . See also Rubik's Cube move notation, where the prime is used to invert moves or move sequences. Notes References External links Unicode General Punctuation code chart Unicode Spacing Modifier Letters code chart Mathematical symbols Typographical symbols