The P&p illusion.

Identical letters can appear to differ in size and shape depending on whether they are interpreted as uppercase or lowercase. The effect is most-dramatic with the letter p. Examination of subjects' estimates of the magnitude of the effect for different stimuli suggests that two factors are involved in the illusion. One factor depends only on whether a letter is interpreted as uppercase or lowercase. This factor can be manipulated by changes in the size or case of the surrounding letters. The second factor, which depends on the possibility of interpreting a vertical line alternatively as an ascender or a descender (as in p or y), seems to involve a change in the perceived size of the letters' loops.