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In the first half of the 20th century, clamp-on square levels (about 3" long) were fairly common. They were more than adequate for normal carpentry tasks like plumbing doors, levelling sills, etc. The Veritas® square level is partially a reincarnation of that earlier tool, but in a far more versatile manner.
First, it attaches to a steel square (steel rule, I-beam, etc.) with integral rare-earth magnets, a much faster attachment method. Second, it has a vertical slot with an integral clamp to hold a ruler 1" to 1-1/8" wide and up to 1/16" thick. This makes it particularly easy to measure the rise and run of just about anything. It can be used to lay out sloping cuts on drywall or to fit molding next to a stair stringer. The tool is also handy as a depth gauge where the reference surfaces are some distance apart, such as on bowl-shaped items.
The body is made of extruded and machined aluminum; the fittings are brass. The bubble level is accurate; bubble movement of 1/16" represents angular change of half a degree. At less than three ounces, it is much handier to carry than a standard level.