Fig. 3
Lines AA, BB, and CC are absolutely straight lines. Lines 1 and 2 are parallel with each other as also are lines 3, 4, 5, 6 and 7. Circle M is of the same diameter as square N.
These statements appear false to the eye; but they are easily proved true by test. This principle of "optical illusion" has been a great factor in the determination of the rules of lettering and in the spacing apart of letters in words.
The principle illustrated in the circle and square of the same diameter (M and N, fig. 3), makes it necessary that the "round" letters be given a greater height than the "square" letters, in order that they may all appear to be of a uniform height. Hence, one of our rules—
RULE 1. All curved portions of letters touching the upper or lower guide line extend beyond.
This rule applies to all styles of lettering. A tendency to exaggerate this distance beyond the guide lines must be guarded against.
5
