A combination [square] is one of the most common modern [tool|tools], one of those that everyone has in their [toolbox]. It is mostly a square, used for determining [right angle|right angles], but it has a bunch of bells and whistles to make it super useful for people who don't need a specialized tool.

The square consists of two primary components; a 'blade', which is essentially a metal [ruler], and a 'head', which is a plastic or metal clamp that attaches to the blade.

The blade is indeed a ruler, usually either 12 inches or 300 millimetres, although other sizes are available. The quality of the blade is highly variable, but is almost always made of [steel]. When removed from the head, it distinguished from the other steel rulers one might have around the house by a groove down the center, used as a guide for the head.

The head is where things start getting fancy. The cheapest combination square you are likely to find will have a plastic head with a [lock nut] to tighten it to the blade, or loosen it to allow it to slide along the blade; the head will have a right angle edge on one side and a 45 degree angled side opposite; and a small, built-in [bubble level]. The normal ones have a metal head and a removable [scriber] tucked into the top. The next step up is to add additional heads, most often a [center square|center head], used to place the square to scribe a line across the end of cylinder so that it will cross the center point; and a [protractor] head, for finding angles other than 45 and 90 degrees.

And that's mostly that. You can find combination squares with notches cut for especially accurate scribing, graduations down to 1/64th of an inch, extra chunky heads (e.g., 2+ inches along all three axis) so that they won't tip when left standing, that can double as a a [saddle square], that have Cool Retro Wooden heads, and etc. It should be noted that a square of the form of a [try square] with a movable head and (usually) none of the combination square's bells and whistles is usually called a [double square].

Combination square (thing)