Set vs. Wind Up

 
The pitcher can initiate their delivery from either the windup position (NF 6-1-2) or the set position (NF 6-1-3). The rules are very different for each position. What determines when the pitcher is in the stretch position and when he is in the windup position?
 
Call
The position of the pitcher's feet determine if he is in the windup position or set position. A pitcher has a pivot foot (right foot for right-handers) and a "free foot" (left foot for right-handers).
 
When pitching from the windup, the pitcher's pivot foot must be in contact with the pitching rubber. His free foot (also called his non-pivot foot) must be anywhere on or behind a line that passes thru the front edge of the pitching rubber.
 
When pitching from the set, the pitcher's pivot foot must in contact with (or immediately in front of) the pitching rubber. His free foot must be in front of the pitching rubber.