I used to have a two-foot steel level with glass vials. It was made in the ’50s. When my son was 3 years old he took his little wooden hammer and knocked out the vials. I remember looking over, seeing what he had done, and his glowing sense of accomplishment. A toy wooden hammer rarely produced such tangible results.

Currently I have three levels. The two-foot level that replaced the one hammered years ago, and two six-footers. Only one of the six-footers is any good. The other I use as a straight edge. The levels I’m talking about are all spirit levels. A spirit level features a bubble of air trapped in a vial with liquid. When the bubble is between the hash marks on the vial, things are plumb or level. Level is when something is perfectly horizontal, like the horizon. Plumb is

90 degrees off of level, like a rocket being launched.

It’s easy to tell a level’s no good if the vial has been hammered, but how do you tell if it works without that kind of clue? To do that you need a wall you can write on. Hold the level against the wall until it reads level. Draw a line on the wall along the top of the level. You’ve just drawn, according to your level, a level line. Next, flip the level over and check if level falls on the same line. Rotate the level and do the same. If you come up with the same line no matter how you rotate the level, the level works. You check plumb in the same manner. Hold the level vertically, draw a plumb line, then rotate the level through all combinations to see if plumb falls on the same line. Framing, combination, and speed squares can all be checked for square in a similar manner. Hold the square against a straight edge and draw a line. If flipping the square yields the same result, your square is square.