Parallel in LaTeX (\parallel)

Where it is used

Relation meaning is parallel to, for lines, planes or vectors; also a parallel-composition operator.

Geometry writes \ell_1 \parallel \ell_2, physics writes components like F_\parallel (often via \|), and electrical engineering uses R_1 \parallel R_2 for resistors in parallel. Process calculi use it for parallel composition, P \parallel Q. The glyph is the same as \|, but \parallel gets relation spacing, so use it between objects and \| (or \lVert, \rVert) for norm delimiters. Its negation is \nparallel. For the KL divergence D(P \parallel Q) the relation spacing is standard, though some prefer \|.

Also listed under: Geometry

Minimal compilable example

Paste this into an empty .tex file and compile it with pdfLaTeX, XeLaTeX or LuaLaTeX.

\documentclass{article}
\begin{document}
Parallel: $\parallel$

\[ D_{\mathrm{KL}}(P \parallel Q) = \sum_x P(x) \log \frac{P(x)}{Q(x)} \]
\end{document}

Related symbols

• Not Parallel\nparallel
• Perpendicular\perp
• Much Less Than\ll
• Superset or Equal\supseteq
• Much Greater Than\gg
• Superset\supset

Frequently asked questions

How do I write Parallel in LaTeX?

Type \parallel in math mode, for example $D_{\mathrm{KL}}(P \parallel Q) = \sum_x P(x) \log \frac{P(x)}{Q(x)}$. It works in plain LaTeX without loading any package.

Which package do I need for \parallel?

None. \parallel is part of standard LaTeX, so it compiles with nothing in the preamble but the document class.

Can I use \parallel in text mode, or only in math mode?

Only in math mode. Outside $...$ LaTeX stops with "Missing $ inserted", so write $\parallel$ in running text. If you only need the glyph in text, paste the Unicode character ∥ instead (with XeLaTeX or LuaLaTeX and a font that has it).