Triangle symbol (△)

How to read it

Henry Africk’s Elementary College Geometry (CUNY) gives △ as the symbol for a triangle and names a triangle by its three vertices in any order, △ABC, △BCA or △CAB. It is shorthand for the word, used in statements such as △ABC ≅ △DEF, “triangle ABC is Congruent symbol (≅) to triangle DEF”. Unicode encodes the outline triangle as WHITE UP-POINTING TRIANGLE (U+25B3) in its geometric shapes block, with “trine” as its other name.

Also searched as: △, △ABC, triangle sign, Δ in geometry.

△ or Δ?

Printed books don’t always use the outline triangle. OpenStax’s Contemporary Mathematics writes Δ ABC ≅ Δ DEF with Δ, the capital Greek letter delta, which looks almost the same. Unicode cross-refers △ to a third look-alike, the increment sign ∆ (U+2206). In a geometry statement, all three mean “triangle”; outside geometry, Δ is the Greek letter, with meanings of its own.