noun
T square
A drafting tool shaped like the letter T, used for drawing horizontal lines and as a guide for triangles.
製図台に丁字定規を当てて水平線を引く。
I draw a horizontal line by placing a T square against the drafting board.
Compound of 丁字 (ていじ, 'T-shaped character') and 定規 (じょうぎ, 'ruler'). The name reflects the tool's T-like shape.