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Anoverline,overscore, oroverbar, is atypographical feature of ahorizontal line drawn immediately above the text. In oldmathematical notation, an overline was called avinculum, 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 inAncient Greek was to indicate compositions ofGreek letters asGreek numerals.^[1] In Latin, it indicatesRoman 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 calledoverstriking, 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 themacron, adiacritical mark placed above (or sometimes below)individual letters. The macron is narrower than the character box.^[2]

In most forms ofLatinscribal abbreviation, an overline ormacron indicates omitted letters similar to use ofapostrophes in English contractions. Letters with macrons or overlines continue to be used inmedical abbreviations in various European languages, particularly forprescriptions. Common examples include

• a,a̅, orā forante ("before")
• c,c̅, orc̄ forcum ("with")
• p,p̅, orp̄ forpost ("after")^[3]
• q,q̅, orq̄ forquisque and its inflections ("every", "each")
• s,s̅, ors̄ forsine ("without")
• x,x̅, orx̄ forexceptus and its inflections ("except")

Note, however, that abbreviations involving the letter h take their macron halfway up the ascending line rather than at the normal height for Unicode overlines and macrons:ħ. This is separately encoded inUnicode with the symbols usingbar diacritics and appears shorter than other overlines in many fonts.

In theMiddle Ages, from the originalIndian decimal writing, before printing, an overline over theunits digit was used to separate the integral part of a number from itsfractional part, as in 9995 (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 99⁹⁵.^{[citation needed]}

In mathematics, an overline can be used as avinculum.

The vinculum can indicate aline segment:^[4]$${\displaystyle \overline{\mathrm{A}\mathrm{B}}}$$The vinculum can indicate arepeating decimal value:$${\displaystyle \frac{1}{7}=0.\overline{142857}=\mathrm{0.142857142857142857142857...}}$$ When it is not possible to format the number so that the overline is over the digit(s) that repeat, one overline character is placed to the left of the digit(s) that repeat:$${\displaystyle 3.\overline{\phantom{I}}3=3.\overline{3}=\mathrm{3.3333333333333333333333333...}}$$$${\displaystyle 3.12\overline{\phantom{I}}34=3.12\overline{34}=\mathrm{3.123434343434343434343434...}}$$Historically, the vinculum was used to group together symbols so that they could be treated as a unit. Today, parentheses are more commonly used for this purpose.

The overline is used to indicate asample mean:^[5]

• ${\displaystyle \overline{x}}$ is the average value of${\displaystyle x_i}$

Survival functions or complementary cumulative distribution functions are often denoted by placing an overline over the symbol for the cumulative:${\displaystyle \overline{F}(x)=1-F(x)}$.

Inset theory and someelectrical engineering contexts,negation operators (also known ascomplement) can be written as an overline above the term or expression to be negated.^[6] For example:

Common set theory notation:

Electrical engineering notation:

in which the times (cross) means multiplication, the dot means logical AND, and the plus sign means logical OR.

Both illustrateDe Morgan's laws and its mnemonic, "break the line, change the sign".

Incommon 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.

${\displaystyle {\log }_{10}0.012\approx -2+0.07918=\overline{2}.07918}$

The overline notation can indicate acomplex conjugate and analogous operations.^[7]

• if${\displaystyle x=a+ib}$, then${\displaystyle \overline{x}=a-ib.}$

In physics, an overline sometimes indicates avector, althoughboldface andarrows are also commonly used:

• ${\displaystyle \overline{x}=|x|\hat{x}}$

Congruence modulon is anequivalence relation, and theequivalence class of the integera, denoted bya_n, is the set{... ,a − 2n,a −n,a,a +n,a + 2n, ...}. This set, consisting of all the integers congruent toa modulo n, is called thecongruence class,residue class, or simplyresidue of the integera modulo n. When the modulusn is known from the context, that residue may also be denoted[a] ora.

Intopology, theclosure of a subsetS of atopological space is often denotedS or${\displaystyle \mathrm{cl}S}$.

Incrystallography, an overline indicates animproper rotation or anegative number:

• ${\displaystyle \overline{3}}$ is theHermann–Mauguin notation for a threefold rotoinversion, used incrystallography.
• ${\displaystyle [\overline{1}1\overline{2}]}$ is the direction withMiller indices${\displaystyle h=-1}$,${\displaystyle k=1}$,${\displaystyle l=-2}$.

Incomputational neuroscience, an overline is used to indicate the "maximal" conductances inHodgkin-Huxley models. This goes back to at least the landmark paper published by Nobel prize winnersAlan Lloyd Hodgkin andAndrew Fielding Huxley around 1952.^[8]

${\displaystyle I_{\mathrm{N}\mathrm{a}}(t)={\overline{g}}_{\mathrm{N}\mathrm{a}}m(V_m{)}^{3}h(V_m)(V_m-E_{\mathrm{N}\mathrm{a}})}$

Overlines are used in subatomic particle physics todenote antiparticles for some particles (with the alternate being distinguishing based on electric charge). For example, theproton is denoted as
p
, andits corresponding antiparticle is denoted as
p
.

Anactive low signal is designated by an overline, e.g.RESET,representing logical negation.

Overlining is also used in graphics design for decoration of text to help convey a message.^[9]

Some Morse codeprosigns can be expressed as two or three characters run together, and an overline is often used to signify this. The most famous is thedistress signal,SOS.

An overline-like symbol is traditionally used inSyriac 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.^[10][11]

WhenMorse code is written out as text, overlines are used to distinguishprosigns and other concatenated character groups from strings of individual characters.

InArabic writing and printing, overlines are traditionally used instead ofunderlines fortypographic emphasis,^[12] 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 ofsyntactic structure. Certain structures are represented by adding an overbar to the unit, as inX. Due to difficulty in typesetting the overbar, theprime symbol is often used instead, as in X′. Contemporary typesetting software, such as LaTeX, hasmade 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.

InHTML usingCSS, overline is implemented via thetext-decoration property; for example,<span>text</span> results in:text.

The text decoration property supports also other typographical features with horizontal lines:underline (a linebelow the text) andstrikethrough (a linethrough the text).

Unicode includes twographic characters,U+00AF ¯MACRON andU+203E ‾OVERLINE. They arecompatibility equivalent to theU+0020  SPACE with non-spacing diacriticsU+0304 ◌̄COMBINING MACRON andU+0305 ◌̅COMBINING OVERLINE respectively; the latter allows an overline to be placed over any character. There is alsoU+033F ◌̿COMBINING DOUBLE OVERLINE. As with anycombining character, it appears in the same character box as the character that logically precedes it: for example, x̅, compared to x‾. A series of overlined characters, for example 1̅2̅3̅, may result either in a broken or an unbroken line, depending on the font.

In Unicode, characterU+FE26COMBINING CONJOINING MACRON is conjoining (bridging) two characters:◌︦◌.

In East Asian (CJK) computing,U+FFE3 ￣FULLWIDTH MACRON is available. Despite the name, Unicode maps this character to both U+203E and U+00AF.^[13]

Unicode maps the overline-like character fromISO/IEC 8859-1 andcode page 850 to theU+00AF ¯MACRON symbol mentioned above. In a reversal of its official name (and compatibility decomposition), it is much wider than an actual macron diacritic over most letters, and actually wider thanU+203E ‾OVERLINE in most fonts. InChromeOS andLinux, the symbol can be added using the keystrokesCtrl+⇧ Shift+U to activateUnicode input, then type "00AF" as the code for the character. On aMac, with theABC Extended keyboard, use⌥ Option+a. InMicrosoft Windows, U+00AF can be entered with the keystrokesAlt+0175 (where numbers are entered from thenumeric keypad).

The Unicode characterU+070F SYRIAC ABBREVIATION MARK is used to markSyriac abbreviations and numbers. However, several computer environments do not render this line correctly or at all.

The Unicode characterU+0B55 ୕ORIYA SIGN OVERLINE is used as a length mark inOdia script.

Collabora Online, an office suite for the web has direct menu support for several styles of Overline in the "Format" menu, with options available under "Format > Character" enabling the default solid Overline to be replaced with double overline, dots, dashes, waves, double waves, and the color of the Overline can be set.

Collabora Office andLibreOfficehave direct menu support for several styles of Overline in the "Format" menu. The user-interface option is available in their word processors on Linux, macOS and Windows, and also in Android (in tablet format), ChromeOS and iPadOS with Collabora Office. Options for Overline are available under the menu: "Format > Character", enabling the default solid Overline to be replaced with double overline, dots, dashes, waves, double waves, and the color of the Overline can be set.

Microsoft Word does not have a menu option to edit Overlining in Microsoft Word for any operating system, or in Word for the web. Overlining can be added using fields codes,EQ \O(). The field code{EQ \O(x,¯)} producesx and the field code{EQ \O(xyz,¯¯¯)} producesxyz; However this does not work in Word on Android, ChromeOS, iPadOS, or Word for the web. In Word 2010 it is necessary to insert an MS Equation object.

Overstriking of longer sections of text, such as in123, can also be produced in manytext processors astext markup as a special form of understriking.

Collabora Online,Collabora Office andLibreOffice have direct menu support for several styles of Overline in the "Format" menu within applications of their office suites, including spreadsheets, presentations and graphics applications. The user-interface option is available in the web based suite, and the locally installable applications for Linux, macOS and Windows, and with Collabora Office in Android (in tablet format), ChromeOS and iPadOS. Options for Overline are available under the menu: "Format > Character", enabling the default solid Overline to be replaced with double overline, dots, dashes, waves, double waves, and the color of the Overline can be set.

InLaTeX, a text <text> can be overlined with$\overline{\mbox{<text>}}$. The inner\mbox{} is necessary tooverride the math-mode (here invoked by the dollar signs) which the\overline{} demands.

Wikimedia Commons has media related toOverlining.

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• Titlo, an overline used to indicate numerals or abbreviations in Cyrillic
• Underscore

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