triangle vs triangular

triangle is a noun but triangular is not a noun.

Word	Noun	Adjective	Verb	Adverb
triangle	Yes	No	No	No
triangular	No	Yes	No	No

triangle (noun)	triangular (noun)
a percussion instrument consisting of a metal bar bent in the shape of an open triangle
any of various triangular drafting instruments used to draw straight lines at specified angles
a three-sided polygon
something approximating the shape of a triangle

triangle (adjective)	triangular (adjective)
	having three angles; forming or shaped like a triangle
	having three sides