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Double hyphen in a Fraktur typeface.

The double hyphen (＝, ゠, or ⸗) is a punctuation mark that consists of two parallel hyphens. It is not to be confused with two consecutive hyphens (--), which usually represents an em dash (—) or en dash (–). In order to avoid its being confused with the equals sign (=), the double hyphen is often sheared slightly.

Usage [link]

The double hyphen is used for several different purposes throughout the world:

• Some typefaces, such as Fraktur faces, use the double hyphen as a glyphic variant of the single hyphen. (With Fraktur faces, such a double hyphen usually is oblique.)
• It may be also used for artistic or commercial purposes to achieve a distinctive visual effect. For example, the name of the Waldorf=Astoria Hotel is officially written with a double hyphen.
• In Merriam-Webster dictionaries if a word is divided at the end of the line, and the division point happens to be a hyphen, it is replaced with a double hyphen to graphically indicate that the divided word is normally hyphenated, for example cross⸗
  country.
• In several dictionaries published in the late nineteenth and early twentieth centuries, all such compound words are linked with double hyphens, whether at the end of the line or not, and the normal use of the single hyphen for non-compound words is retained. An example from the first or second page of such dictionaries is Aaron's⸗rod. Examples include the Century Dictionary and Funk & Wagnalls New Standard Dictionary of the English Language.
• It is used as a distinct punctuation mark by Coptic language scholars.
• It is used as a distinct puncutation mark in Ojibwe language publications in the Fiero orthography, as a hyphen is used to separate compound preverb units, while a double hyphen is used to divide a word at the end on a line. However, due to lack of availability of a double hyphen in most fonts, an equal sign is often used as a substitute.
• In Japanese, the double hyphen (ダブルハイフン, daburu haifun^?) in rare cases replaces an English en dash or hyphen when writing foreign words in katakana due to their potential confusion with the prolonged sound mark (ー). It is generally used to separate multiple foreign names, as in the cases of the Russell–Einstein Manifesto (ラッセル゠アインシュタイン宣言^?) and The Waldorf=Astoria Collection (ウォルドルフ゠アストリア^?). It may also be used to separate a person's given and family names or to separate other foreign words. The middle dot (·) is however much more commonly used for these purposes. The double hyphen is part of the JIS X 0213 standard, but is not included in more commonly used character encodings, such as Shift-JIS and EUC-JP. For this reason, the equals sign is frequently used in its place.

Unicode [link]

When the double hyphen is used as a functionally equivalent graphical variant of the single hyphen, it is represented in Unicode as a normal hyphen.

When used as a punctuation mark distinct from the single hyphen, the double hyphen is represented in Unicode by two different code points. A third code point has been assigned for the use of a generic (non-Asian) double hyphen at U+2E40, though this character is still under ballot and has not been formally adopted in the standard.