What Is Stem?
A stem is the main vertical or diagonal stroke of a letter — the upright of an "l," the two diagonals of a "V," the spine of an "H." Stem thickness, and how it contrasts with thinner strokes, is a defining feature of a typeface's weight and style.
The relationship between thick stems and thin connecting strokes is called stroke contrast. High contrast (as in Didone serifs) looks elegant; low contrast (as in many sans-serifs) looks sturdy and even.
Consistent stem widths across a typeface give text an even colour — the uniform grey tone a block of well-set type produces when seen from a distance.