Some research suggests that G-spot and clitoral orgasms are of the same origin. Masters and Johnson were the first to determine that the clitoral structures surround and extend along and within the labia. Upon studying women's sexual response cycle to different stimulation, they observed that both clitoral and vaginal orgasms had the same stages of physical response, and found that the majority of their subjects could only achieve clitoral orgasms, while a minority achieved vaginal orgasms. On this basis, Masters and Johnson argued that clitoral stimulation is the source of both kinds of orgasms, reasoning that the clitoris is stimulated during penetration by friction against its hood.