Two female partners rub their vulvas together until orgasm. Tribadism done with both partners lying on their backs, genitals facing each other, is known as scissors or double Y. It can also be done with one partner on her back and the other in a kneeling position with one leg between the other's legs and the other outside; this usually requires one of the legs of the lying-down partner to be held up - preferably on the shoulder of the kneeling partner.
Also called tribbing, scissoring or donut bumping.