Triangular

Nuttall's Standard Dictionary of the English Language · 1890 · p. 187

tri-ang' -gu-lar, a. having three angles; in the form of a triangle. Triangular numbers, the series of numbers formed by the succe ive sums of the terms of an arithmetical progre ion, of which the common difference is 1. Triangular compa es, compa es with three legs, used in the construction of maps, charts, Triangularly, tri-ang'-gu-lar-le, ad. after the form of. a triangle.