Rule 5-15 (pg. 24)
Section 15 - Inbounds
Article 1 ... Contestants are considered to be inbounds if the supporting parts of either wrestler are inside or on the boundary lines.
Article 2 ... Supporting points are the parts of the body touching, or within, the wrestling area which bear the wrestler's weight, other than those parts with which the wrestler is holding the opponent.
a. When down on the mat the ususal points of support are:
1. the knees;
2. the side of the thigh;
3. the buttocks;
4. the hand(s).
b. When the defensive wrestler is on his/her backwhile the supporting parts of either wrestler are inbounds, wrestling shall continue as long as there is a possibility of the offensive wrestler bringing the opponent back into the inbounds area. In this situation any part of a defensive wrestler's shoulder or scapula is considered to be the supporting parts.
c. Near-fall points or a fall shall be earned only while any part of both defensive wrestler's shoulders or boths scapulae are inbounds.
Article 3 ... Wrestling shall continue as long as the supporting parts of either wrestler remain inbounds. If there is no action at the edge of the mat and one wrestler is out of bounds, the referee may stop the match.