The LaTeX command for parallel (∥) is \parallel.
\parallelWriting a paper? The table generator and the BibTeX cleaner are one click away, no account.
Write \parallel inside math mode, that is between $…$ for inline math or in a \[ … \] or equation block for display math. It is part of core LaTeX math, so no extra package is required.
As a relation, \parallel gets a thick space on each side, and a long inline formula may break after it. With unicode-math under XeLaTeX or LuaLaTeX, typing ∥ in a formula gives the same symbol.
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Use \parallel in math mode; it needs no extra package. For example, $a \parallel b$.
None: \parallel is built into LaTeX, so no extra package is required.