Definition: The missionary posture is a sexual posture proceding from the kamasutra and common between humans, even though is also practized among some animal species, like bonobos and armadillos.
In its seminal study "Sexual Behaviour in Women" (1953), the researcher Alfred Kinsey declared that a 91% of married women practized this posture the most, while a 9% said that they adopted this position while fucking.
The missionary posture consists essentially in laying face to face, the fucked person down with their legs open. The legs can be laying horizontally, lifted to the tits or stick around the man, which can be done in different hights with one or both legs: behind the legs of the man fucking, on his butt or over his shoulders.
Usually, the tallest, the deepest will be anal or coital penetration. The man fucking is on the womam with his legs between her legs and the penis is at the same level as the penetration hole (anus or vagina).