The plumb rule was a straight board with a plumb bob hanging from the top. The mason held the board against the wall and watched the bob. If the bob hung parallel to the board the wall was plumb. If the bob swung away from the board the wall was leaning. The plumb rule was the oldest level in the world. Gravity was the mechanism. The string was the indicator. The board was the reference. The mason was the judge. The whole instrument cost nothing. A board and a string and a weight. The earth did the rest.

The plumb rule was four feet long. Some were six. The longer the rule the more accurate the reading because a small lean showed up bigger on a long board. A quarter inch of lean on a four-foot rule was obvious. A quarter inch of lean on a one-foot rule was hard to see. The mason used the long rule for the first reading and the short rule for adjustments. The long rule said this wall is leaning. The short rule said this brick is the one that is leaning. The mason tapped the brick with the handle of the trowel and checked again. Tap and check. Tap and check. The wall straightened one brick at a time.

The ancient Egyptians used plumb rules. The Romans used plumb rules. The medieval masons used plumb rules. The mason on your block in nineteen fifty used the same plumb rule the Egyptians used three thousand years before him. The technology did not change because the technology could not be improved. Gravity did not update. The string did not need a battery. The weight did not need calibration. The plumb rule was perfect the day it was invented and it was still perfect the day the laser replaced it and the laser was no more accurate than the string. The laser was faster. The laser was easier to read. But the laser was not more accurate because accuracy was set by gravity and gravity had not changed since the Egyptians.

The laser level projects a line. The plumb rule hangs a line. The projected line requires power. The hanging line requires weight. The projected line disappears when the battery dies. The hanging line disappears when you pick up the weight. The mason who carried a plumb rule carried the whole history of vertical in his tool bag. The mason who carries a laser carries a machine. Both men build a straight wall. One man knows why it is straight. The other man knows that it is straight. The difference is the string.

See also: Plumb Bob, Spirit Level