So many beautiful tools in the world! (And I'm in the dark about how to use most of them!) I would guess this old brass triangle was used by a carpenter, or a framer (?), through would certainly look very lovely now sitting atop a pile of papers on a drafting table, or a studio table now. I especially love the the raised rectangle near the 30-degree angle, which would allow one to rest a ruler against it to draw a straight line in the opposite direction, which I assume was the point, though it also almost seems as if a mend. Very substantial and with a gorgeous patina, including lots of straight scratches to the surface, so surely whoever used it frequently had a blade in hand while using it. 

Hypotenuse measures 13 3/8". Other sides: 11 3/4"; 6 1/4". Very good condition, all the more beautiful for the age and wear.