Combination squares lay out 90° angles with their anvil, lay out 45° angles with their shoulder, and check level and plumb with their bubble vials. Also known as combo squares, these multifunctional tools assist with gauging flatness, aligning tool blades and fences, setting bevel angles on saws and jointers, striking long lines on workpieces, and more. They include a square head but also accept other types of compatible heads to perform additional measurement and layout tasks.

Combination square sets include interchangeable heads for different measurement and marking tasks. Their square head lays out 90° angles with its anvil, lays out 45° angles with its shoulder, and checks level and plumb with its bubble vials. It assists with gauging flatness, aligning tool blades and fences, setting bevel angles on saws and jointers, striking long lines on workpieces, and more. Their other heads perform more specialized functions such as finding center and measuring a large range of angles.

Double squares have a right-angle reference on each side of their sliding head, so there's no need to turn over the blade or take two sets of measurements. They check and lay out pairs of 90° angles for more precise material alignment and machine setups than combination squares. Common uses include confirming squareness of corners and cut edges, checking alignment of band saw blades, gauging height and depth, and positioning work for assembly and glue-ups.