Originally Posted by
davidad
The 31.8 is the diameter of the bulge in the center.
Correct. The stem you buy must specify it's for 31.8 mm bars too since the stem's handlebar clamp will go around the bar at that center bulge.
From the center bulge, the bars will taper down to the standard 23.8 mm diameter where the brake levers or brifters are placed on road bars or 22.2 mm for where the brake and shift levers mount on MTB bars.