Main Page | See live article | Alphabetical index A T-square is primarily a guide for drawing horrizontal lines on a drafting table. It is also used to guide the triangle that draws vertical lines. It's name comes from the general shape of the instrument where the horizontal member of the T slides on the side of the drafting table. Common lengths are 18", 24", 30", 36" and 42". This article is from Wikipedia. All text is available under the terms of the GNU Free Documentation License.