example, the proton is denoted as , and its corresponding antiparticle is denoted as . Engineering An active low signal is designated by an overline, e.g. , representing logical negation. Morse (CW) 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, . Writing 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. Linguistics 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. Implementations HTML with CSS 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). Unicode Unicode includes two graphic characters, and . They are compatibility equivalent to the with non-spacing diacritics and respectively; the latter allows an overline to be placed over any character. There is also . As with any combining 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