The missionary position allows easy entry into the vagina. The man can use force and gravity to help himself in, and the position allows the woman to relax her vaginal muscles and help her partner guide his penis gently in with her hand. Suzi Godson said, "In an observational study carried out in The Netherlands, magnetic resonance imaging was used to study the male and female genitals during coitus. The images illustrate the very natural fit of the male and female genitals in missionary position. The penis has the shape of a boomerang—one third of its length consists of the root of the penis—and the vaginal walls wrap snugly around it."