| In typography, the invisible floor on which (most) letters in a line of type sit is called the baseline. The leading between lines of text is the vertical distance between baselines.
Some letters fall below the baseline. Of course those with descenders (most often gjpqyQ) fall below the line. What may not be so obvious is that font designers take care to try and preserve the baseline optically rather than mathematically. To that end, the bottoms of letters with bowls (abcdegopqsBCGJOQSU, sometimes D) actually dip slightly below the baseline.
An optically even baseline is the foundation for clear boumae and thereby part of easy reading. The baseline of bouma is reinforced by serifs, and bracketed serifs even moreso.
Baseline font fun fact: the font Hobo is the only mainstream font with a virgin baseline. It has no descenders and even its bowled letters square up perfectly.
See also capline, x-height | 
