This function, along with the next two, determines how Calamus will measure the height of a character. Only one of the three icons may be selected at a time.
Em-height is the height of a complete line. It corresponds to the
height of the lead type once used for typesetting, which also had to
include space for descenders. When this lead type was set without
extra lead between lines (
leading), the em-height was exactly
the same as the line spacing. For this reason it is also often called