Answer:
There is an optical illusion well known to all designers, in which a horizontal line drawn across the middle of a rectangle appears to be below the middle. In order that the divisions may seem to be symmetrical such a line must be drawn above the middle. In the construction of letters this illusion must be provided for in what may be called the "rule of stability." In order to give the appearance of stability such letters as the B E K S X and Z, with the figures 3 and 8 must be drawn smaller at the top than the bottom. To see the effect of this illusion turn a printed page upside down and notice the letters mentioned.
