Combination Square

A combination square slides a ruled steel blade through an adjustable head, most commonly cast with both a 90-degree and a 45-degree reference face, and usually including a small spirit level and a scribing point built into the head. Loosening and sliding the head along the blade adjusts and locks depth or width settings, and the blade itself can be removed entirely to use as a plain steel rule.

That versatility -- square, 45-degree miter check, depth gauge, and rule in one tool -- is exactly what distinguishes it from the more purpose-built Speed Square (fast rough layout and roof framing) or a fixed carpenter's square (checking and marking large right angles); a combination square trades some of their specific speed and reach for genuine multi-purpose flexibility at the bench.