What is "double hyphen"

The double hyphen (＝ or ゠) is a punctuation mark that consists of two parallel hyphens. It was a development of the earlier (⸗) which developed from a Central European variant of the virgule slash, originally a form of scratch comma. In order to avoid its being confused with the equals sign (=), the double hyphen is often given as double oblique hyphen in modern typography. The double hyphen is also not to be confused with two consecutive hyphens (-em dash (—) or en dash (–); that practice arose due to the limitations of typewriter character sets which did not have both hyphen and dash.