A T-shaped ruler used in technical drawing. Rare term; the more common word is Ｔ定規 (ティーじょうぎ).
See also: Ｔ定規
I used a T-square in drafting class.
More common term for T-square. 丁字形定規 is a rare, more descriptive alternative.
Compound of 丁字形 (ていじけい, 'T-shaped') and 定規 (じょうぎ, 'ruler').