The combination square is adjustable, consisting of a rigid steel rule, typically 12 inches long, with a headpiece that slides along its length. The headpiece has both a 90-degree edge and one that forms a 45-degree angle with respect to the rule. It’s ideal for marking (and checking) both 90-degree crosscuts and miter cuts. A bubble level in its handle makes the tool useful for leveling as well.
Related: Tool Tip—Toolboxes