It was a development of the earlier double oblique hyphen ⸗, which developed from a Central European variant of the virgule slash, originally a form of scratch comma.
In order to avoid it being confused with the equals sign =, the double hyphen is often shown as a double oblique hyphen in modern typography.
The double hyphen is also not to be confused with two consecutive hyphens (--), which are often used to represent an em dash — or en dash – due to the limitations of typewriters and keyboards that do not have distinct hyphen and dash keys.
The double hyphen is used for several different purposes throughout the world: When the double hyphen is used as a functionally equivalent graphical variant (allograph) of the single hyphen, it has the same Unicode code point as a conventional hyphen (since how it is displayed/printed is a font choice on that occasion).
Other forms of double hyphen are given unique codepoints in Unicode: