How to type Double Vertical Line

‖ lives in the General Punctuation block (U+2000-U+206F), dashes, quotation marks, spaces and reference marks used across languages. Within Unicode it is classified as Po, a punctuation mark.

What Double Vertical Line is used for

In mathematical notation ‖ is a fence: it encloses or separates rather than acting on anything, and stretches to the height of what it sits beside. That is the classification unicode-math gives it, and it is what decides how the symbol is set: $\Vert$ in LaTeX, in maths mode rather than in a sentence.

What Unicode says about Double Vertical Line

Unicode annotates ‖ directly. Against U+2016 the code chart carries the note “Used in pairs to indicate norm of a matrix”. That is the standard saying what the character is for, rather than an inference from its name.

The code chart points from ‖ to Parallel To, combining double vertical stroke overlay (U+20E6) and double vertical bar (U+23F8). These are the characters the standard thinks you might have meant instead, which makes them the ones worth checking when Double Vertical Line looks nearly right but not quite.

How Double Vertical Line behaves in software

‖ is East Asian Ambiguous: one column in a Western locale and two in a CJK one. That is why a table that lines up on your machine can break on somebody else's, with no change to the text at all. It carries no direction of its own, so in a line that mixes English with Arabic or Hebrew it takes the direction of the text around it rather than forcing one.

Every way to write Double Vertical Line

If ‖ arrives as ‖

Its three UTF-8 bytes E2 80 96 are being read one at a time as Windows-1252. The text is undamaged; the reader has to be told it is UTF-8.