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A double hyphen is a rarely used punctuation mark which resembles a standard hyphen but utilizes two parallel hyphens. The symbol itself for this mark can appear horizontally as “=” in some instances, though this can create confusion since it appears to be the same symbol used in mathematical equations for “equals.” In order to avoid this confusion, this symbol can also be created at a slight upward slant. A double hyphen should not be confused with a dash.
The use of a double hyphen is typically quite rare, as its purpose and meaning are not often considered in the English language. One of the most common uses of this symbol over a standard hyphen is for a word or logo to appear more unique and unusual, though in meaning it is essentially the same as a standard hyphen. A standard hyphen is typically used for compound words in which two or more words are not completely combined but remain separate through a hyphen. Words such as “part-time” and “long-term” both utilize a hyphen and could be written with a double hyphen as “part=time” or “long=term.”
A double hyphen can also be used in some written works to indicate that a hyphen that would normally appear at the end of a sentence also appears within the word itself. For example, if a hyphenated word such as “part-time” appeared at the end of a sentence without enough room for the entire word, it might be split between the end of one line and the start of the next. When a word is split this way, a hyphen is typically used to indicate that it continues on the next line and is still part of the same word. In an instance where this hyphen aligns with an existing hyphen, such as a line in which “part-time” was split with “part” at the end of one line and “time” at the beginning of the next, a double hyphen could be used to indicate to the reader that both uses of a hyphen are relevant.
The term “double hyphen” is also sometimes used to refer to a symbol in written text that is actually called a dash. While a hyphen and a dash both appear quite similar, they are used to express different meanings and for different purposes. A short dash, or “en dash” because it is meant to be the width of the letter “n,” is used to express a range such as “20-30 loaves of bread were baked today.”
There is a longer dash, called an “em dash,” which is the width of the letter “m,” used to break up clauses in a sentence. For example, it can be used instead of commas in “The man walked into the kitchen — his thoughts had been elsewhere and he required a moment to remember why he was there — before he opened the oven and took out the cookies.” This longer dash can sometimes be expressed as a sign that appears as “--” that may be incorrectly referred to as a double hyphen, though it is really a double dash or em dash.