The chordae tendineae are attached to papillary muscles that cause tension to better hold the valve. Together, the papillary muscles and the chordae tendineae are known as the subvalvular apparatus. The function of the subvalvular apparatus is to keep the valves from prolapsing into the atria when they close. The subvalvular apparatus has no effect on the opening and closure of the valves, however, which is caused entirely by the pressure gradient across the valve. The peculiar insertion of chords on the leaflet free margin, however, provides systolic stress sharing between chords according to their different thickness.