Overline

Horizontal line immediately above a portion of writing

Description Sample Unicode CSS/HTML Overline(markup) text decoration: overline; Overline(character) ‾ U+203E ‾, ‾ X̅x̅ (combining) U+0305 X̅ Double overline(markup) Xx text decoration: overline; text decoration style: double; Double overline(character) X̿x̿ (combining) U+033F X̿ Macron(character) ¯ U+00AF ¯, ¯ X̄x̄ (combining) U+0304 X̄ X̄x̄ (precomposed) varies
An overline, overscore, or overbar, is a typographical feature of a horizontal line drawn immediately above the text. In old mathematical notation, an overline was called a vinculum, a notation for grouping symbols which is expressed in modern notation by parentheses, though it persists for symbols under a radical sign. The original use in Ancient Greek was to indicate compositions of Greek letters as Greek numerals. In Latin, it indicates Roman numerals multiplied by a thousand and it forms medieval abbreviations (sigla). Marking one or more words with a continuous line above the characters is sometimes called overstriking, though overstriking generally refers to printing one character on top of an already printed character. An overline, that is, a single line above a chunk of text, should not be confused with the macron, a diacritical mark placed above (or sometimes below) individual letters. The macron is narrower than the character box.

In the Middle Ages, from the original Indian decimal writing, before printing, an overline over the units digit was used to separate the integral part of a number from its fractional part, as in 995 (meaning 99.95 in decimal point format). A similar notation remains in common use as an underbar to superscript digits, especially for monetary values without a decimal separator, as in 99.

In common logarithms, a bar over the characteristic indicates that it is negative—whilst the mantissa remains positive. This notation avoids the need for separate tables to convert positive and negative logarithms back to their original numbers.

Congruence modulo n is an equivalence relation, and the equivalence class of the integer a, denoted by , is the set { , a − 2n, a − n, a, a + n, a + 2n, }. This set, consisting of all the integers congruent to a modulo n, is called the congruence class, residue class, or simply residue of the integer a modulo n. When the modulus n is known from the context, that residue may also be denoted [a] or .

In topology, the closure of a subset S of a topological space is often denoted or .

In computational neuroscience, an overline is used to indicate the "maximal" conductances in Hodgkin Huxley models. This goes back to at least the landmark paper published by Nobel prize winners Alan Lloyd Hodgkin and Andrew Fielding Huxley around 1952.

Overlines are used in subatomic particle physics to denote antiparticles for some particles (with the alternate being distinguishing based on electric charge). For example, the proton is denoted as , and its corresponding antiparticle is denoted as .

An active low signal is designated by an overline, e. g. , representing logical negation.

Some Morse code prosigns can be expressed as two or three characters run together, and an overline is often used to signify this. The most famous is the distress signal, .

An overline like symbol is traditionally used in Syriac text to mark abbreviations and numbers. It has dots at each end and the center. In German it is occasionally used to indicate a pair of letters which cannot both be fitted into the available space. When Morse code is written out as text, overlines are used to distinguish prosigns and other concatenated character groups from strings of individual characters. In Arabic writing and printing, overlines are traditionally used instead of underlines for typographic emphasis, although underlines are used more and more due to the rise of the internet.

X bar theory makes use of overbar notation to indicate differing levels of syntactic structure. Certain structures are represented by adding an overbar to the unit, as in Due to difficulty in typesetting the overbar, the prime symbol is often used instead, as in X. Contemporary typesetting software, such as LaTeX, has made typesetting overbars considerably simpler; both prime and overbar markers are accepted usages. Some variants of X bar notation use a double bar (or double prime) to represent phrasal level units. X bar theory derives its name from the overbar. One of the core proposals of the theory was the creation of an intermediate syntactic node between phrasal (XP) and unit (X) levels; rather than introduce a different label, the intermediate unit was marked with a bar.

In HTML using CSS, overline is implemented via the text decoration property; for example, <span style="text decoration: overline">text</span> results in:
The text decoration property supports also other typographical features with horizontal lines: underline (a line the text) and strikethrough (a line the text).