An overbar over a letter is a traditional way of specifying certain Latin abbreviations. For example, s̅ (s overbar) stands for Latin, sine (meaning without), c̅ (c overbar) is an abbreviation for Latin, cum (meaning with), ā (a overbar) stands for Latin, "ante" (meaning "before"), and p̄ (p overbar) stands for Latin, "post" (meaning "after").
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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.
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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.
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Sum and product commute and associate, as in the usual algebra of real numbers. As for the order of operations, brackets are decisive if present. Otherwise '∙' precedes '+'. Hence A∙B + C is parsed as (A∙B) + C and not as A∙(B + C). Complementation is denoted by writing an overbar over its argument.
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Complement component 1s (, C1 esterase, activated complement C1s, complement C overbar 1r, C1s) is a protein involved in the complement system. C1s is part of the C1 complex. In humans, it is encoded by the C1S gene. C1s cleaves C4 and C2, which eventually leads to the production of the classical pathway C3-convertase.
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Complement C1r subcomponent (, activated complement C1r, C overbar 1r esterase, C1r) is a protein involved in the complement system of the innate immune system. In humans, C1r is encoded by the C1R gene. C1r along with C1q and C1s form the C1 complex, which is the first component of the serum complement system. C1r is an enzyme that activates C1s to its active form, by proteolytic cleavage.
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The prime can be used in the transliteration of some languages, such as Slavic languages, to denote palatalization. Prime and double prime are used to transliterate Cyrillic yeri (the soft sign, ь) and yer (the hard sign, ъ). However, in ISO 9, the corresponding modifier letters are used instead. Originally, X-bar theory used a bar over syntactic units to indicate bar-levels in syntactic structure, generally rendered as an overbar.
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An antideuteron is the antimatter counterpart of the nucleus of deuterium, consisting of an antiproton and an antineutron. The antideuteron was first produced in 1965 at the Proton Synchrotron at CERN and the Alternating Gradient Synchrotron at Brookhaven National Laboratory. A complete atom, with a positron orbiting the nucleus, would be called antideuterium, but antideuterium has not yet been created. The proposed symbol for antideuterium is , that is, D with an overbar.
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A bar or stroke is a modification consisting of a line drawn through a grapheme. It may be used as a diacritic to derive new letters from old ones, or simply as an addition to make a grapheme more distinct from others. It can take the form of a vertical bar, slash, or crossbar. A stroke is sometimes drawn through the numerals 7 (horizontal overbar) and 0 (overstruck foreslash), to make them more distinguishable from the number 1 and the letter O, respectively.
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By 130 AD, Ptolemy, influenced by Hipparchus and the Babylonians, was using a symbol for 0 (a small circle with a long overbar) within a sexagesimal numeral system otherwise using alphabetic Greek numerals. Because it was used alone, not as just a placeholder, this Hellenistic zero was the first documented use of a true zero in the Old World. In later Byzantine manuscripts of his Syntaxis Mathematica (Almagest), the Hellenistic zero had morphed into the Greek letter Omicron (otherwise meaning 70). Another true zero was used in tables alongside Roman numerals by 525 (first known use by Dionysius Exiguus), but as a word, meaning nothing, not as a symbol.
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