Number of positions

This man goes to give a lecture on Human Sexual Behaviour, and the hall is packed. He explains that after a long study of all the worlds literature he has concluded that there are 35 positions for sexual intercourse.
A voice from the back of the hall shouts “36”

The lecturer says “I am afraid you are wrong, my studies indicate quite clearly there are only 35 positions. The commonest of these is the Missionary position with the man on top of the woman.

The voice from the back of the hall shouts “37”