Of these, the first three require two values separated by an "interval designator" which is usually a solidus (more commonly referred to as a forward slash "/"). Section 3.2.6 of ISO 8601-1:2019 notes that "A solidus may be replaced by a double hyphen ["--"] by mutual agreement of the communicating partners.", and previous versions used notations like "2000--2002". Use of a double hyphen instead of a solidus allows inclusion in computer filenames. A solidus is a reserved character and not allowed in a filename in common operating systems.