The basic missionary position involves one partner (portrayed here by our model B) lying on their back while the other, (model A) lies on top of them, using their upper body to support their weight and movement. Traditionally, and sex positions don't get more traditional than this one, the partner on top is creating the movement by thrusting with their hips.
But the basic missionary doesn't require the person on top to do all the work. The person on the bottom can be moving, wriggling, and even thrusting their hips upward. And as you'll see in the variations to follow, just slightly altering the basic position can make this a more comfortable and equal action position.
The nicest part of the basic missionary is the face to face closeness it offers. But the trade off is a position that requires a lot of strength from one person and an angle of penetration that simply isn't that great for most.
