A drafting square or T square is a square that is fixed in the shape of a T. This primarily used by draftsmen who need to make straight lines on technical drawings, and as such is designed only to make straight lines and find right angles (with the help of a set square) in 2 dimensions. Because these generally lie across a drafting table they are generally long enough to reach all the way across the table, meaning that these are some of the longest squares you are likely to see (although L squares also get big). It's not uncommon to see the long leg reaching a meter or even more. Because only the long leg is used for a straight edge, the top of the T is often formed in a tapered shape, with the center point where the blade connects to the head being thick for the attachment, but then tapering or curving into points at the end, preventing moving corners from catching on anything. The long blade is often wood or metal, but is usually edged in clear plastic so that you can see what you are drawing on.
T squares are also used to measure, scribe, and cut drywall. These squares are usually made of aluminium and are 48-inches (or 1,200 mm) down the blade. These are also much more likely to have a 'lopsided' head, with one arm longer than the other, e.g. 6 inches on one side and 16 on other.