Perpendicular symbol (⊥)

How to read it

OpenStax’s Contemporary Mathematics defines perpendicular lines as lines that meet at a 90° angle, writes l₁ ⊥ l₂, and marks the right angle itself on a diagram with a little square in the corner. Henry Africk’s Elementary College Geometry (CUNY) gives the same sign and writes it between the names of two lines.

Also searched as: ∟, ⊥, ⟂, perpendicular sign, right angle symbol, upside down T.

⟂ or ⊥?

Unicode has two characters for the shape. ⟂ PERPENDICULAR (U+27C2), “orthogonal to”, is the relation between two lines, and its chart notes that it is typeset with extra space on each side. ⊥ UP TACK (U+22A5) is the same shape as a symbol in its own right, logic’s “bottom”, a statement that is always false.

On engineering drawings

On a technical drawing, ⟂ in a feature control frame is the perpendicularity tolerance. The UK National Physical Laboratory’s guide defines its zone as two planes or two lines perpendicular to a datum, which the surface or axis must lie between. In geometry, AB ⊥ CD is an exact statement about two lines; on a drawing, the symbol limits how far from exact a real part may be.