brigidm (asker) May 16, 2012:

Thanks, Joachim. I'm posting some additional info and a link for future reference: "An em is a unit of measurement in the field of typography, equal to the currently specified point size.
The name of em is related to M. Originally the unit was derived from the width of the capital "M" in the given typeface.[1]
This unit is the same for all fonts at a given point size."
http://en.wikipedia.org/wiki/Em_(typography)