Compass will convert the font sizes from px to em and fit it into the vertical rhythm automatically with the necessary margin A neat trick, for sure. More on using the vertical rhythm properties and mixins: http://compass-style.org/reference/compass/typography/vertical_rhythm/ Note: The line height is relative to the constant $base-line-height. The number you specify for the leader, trailer and leading will be multiplied by the constant, then converted to its relative value in ems. Don’t worry about nested elements etc, Compass will do the maths for you :)