Parallel (∥) in LaTeX
\parallel produces the parallel symbol (two vertical bars, ∥), the standard relation for saying two lines, segments or planes are parallel.
LaTeX command
\parallelNot the symbol you meant? Draw it to find the command →
In geometry \parallel sits between two objects exactly like an equals sign sits between two quantities: \ell_1 \parallel \ell_2 reads as 'line one is parallel to line two'. It is a relation, not a delimiter, so LaTeX spaces it correctly on both sides without any extra tweaking.
The same glyph does double duty elsewhere in mathematics. It appears as shorthand for concatenation in computer science, and in some probability and logic texts as an informal 'independent of' marker, though the more standard symbol for statistical independence is the doubled perpendicular sign, \perp\!\!\!\perp, not \parallel.
Keep \parallel separate in your head from a norm. A norm such as the length of a vector should use \lVert x \rVert (paired delimiters that grow with \left and \right), not two typed pipe characters, which give poor spacing and cannot resize automatically.
Examples
$\ell_1 \parallel \ell_2$two parallel lines
$AB \parallel CD$parallel segments inside a geometry proof
$a \nparallel b$a is not parallel to b (needs amssymb)
$P \parallel Q \parallel R$chaining three mutually parallel objects
Tips and common mistakes
- Use
\nparallel, not a hand-built slash through\parallel, for 'not parallel'; it needs the amssymb package. \paralleland \| render the same glyph, but only\parallelcarries built-in relation spacing, so prefer it when it functions as a relation between two named objects.- For vector or function norms use
\lVertx\rVertrather than\parallelor a bare double pipe; norms are delimiters, not relations, and need to resize with their contents. - In a single proof that uses both perpendicular and parallel lines, pairing
\perpand\parallelconsistently makes the logic easier to scan. - Statistical independence is conventionally
\perp\!\!\!\perp, not\parallel, even though some informal notes borrow the parallel bars for the same idea.
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What is the LaTeX code for the parallel symbol?
\parallel, which renders as ∥. The plain double pipe \| gives the same glyph but without proper relation spacing.
How do I write 'not parallel' in LaTeX?
Use \nparallel, which renders as ∦. It requires the amssymb package.
What's the difference between \parallel and \|?
They draw the same two vertical bars, but \parallel is defined as a mathematical relation with spacing on both sides, so it is the right choice between two objects such as .
Is \parallel used for vector norms?
No. Norms should use \lVert x \rVert, which is a pair of growable delimiters, not a fixed relation symbol like \parallel.