A ruler or guide used in mechanical and architectural drawing. (noun)
The Century Dictionary (Public Domain)
Use "t-square" in a sentence
"Fallene packs her bag and her t-square and goes back to the real world, and sew it goes!"
"It's our go-to weapon, our primary tool of choice: kind of like our t-square/hammer/calculator/Singer sewing machine, all rolled into one."
"I only use it once in a while, but I still have the metal-edged drawing board my parents gave me for Christmas the year I was in 7th grade (1962), along with a t-square and triangles (one of which I still have), to encourage my interest in becoming an architect."