A lot vs. Alot: 9 Grammatical Pitfalls
late 14c., from Old French triangle (13c.), from Latin triangulum "triangle," from neuter of adj. triangulus "three-cornered," from tri- "three" (see tri-) + angulus "corner, angle" (see angle (n.)).
In the huts of witches all the instruments and implements are triangular. ["Handwörterbuch des deutschen Aberglaubens"]
triangle tri·an·gle (trī'āng'gəl)n. A three-sided area, space, or structure.