I may have just sent a bit -- accidental, sorry.
Most T-squares have a bit of a lip on the 'T', where it attaches to the long part. This lip catches the edge of your drawing table, or cutting mat, keeping it perpendicular to the edge.
Usually this type of 'T-square' comes from an art supply store and is used for technical drawing, engineers or architects. They are often wood-ish with plastic edges on the long part and the 'T' ion mine is black.
Mary