A T-square is a drawing tool. It is a long straight ruler with a short piece fixed across one end, so the whole thing looks like a capital T. You hook the short crosspiece over the left edge of your drawing board, and then the long arm lies flat across the paper at a perfect right angle to that edge. Slide it up or down, and every line you draw is exactly parallel to the last one. That is the whole point: it is not for measuring, it is for keeping lines straight and square when a hand alone would wobble. Architects, engineers and design students used them constantly before computers, together with a set square (a triangle) laid on top of the T-square to get vertical and slanted lines. Today the word feels a little old-fashioned in daily work — most plans are drawn on a computer — but it is still standard in art rooms, technical drawing classes, and anywhere people draw by hand. It is a countable noun: one T-square, two T-squares. The capital T is kept in the spelling, and the hyphen is normal. You may also see the shape used loosely in other places (a 'T-square' pattern of streets, for example), but that is a stretch from the tool. Note that it is a whole object with a name, not two words 'T' and 'square' put together for the moment.