We were surprised by this result. It turns out that paragraph spacing (i.e. the
space between the last line of one paragraph and the first line of the next)
rarely equals the leading (which would be the main characteristic of perfect
vertical rhythm). More often, paragraph spacing is just 75% of the
paragraph leading. The reason may be that leading usually includes the
space taken up by descenders, and because most characters do not have
descenders, additional white space is created under the line.
AIGA is a perfect example of optimal leading. Its font size is 13.21 pixels (converted
from ems) and its line height is 19.833 pixels (conversion from ems). In fact,
19.8333 ÷ 13.2167 = 1.5011.