The G-spot, also called the Gräfenberg spot (for German gynecologist Ernst Gräfenberg), is characterized as an erogenous area of the vagina that, when stimulated, may lead to strong sexual arousal, powerful orgasms and potential female ejaculation. It is typically reported to be located up the front (anterior) vaginal wall between the vaginal opening and the urethra and is a sensitive area that may be part of the female prostate. The existence of the G-spot has not been proven, nor has the source of female ejaculation. Although the G-spot has been studied since the 1940s, disagreement persists over its existence as a distinct structure, definition and location. The G-spot may be an extension of the clitoris, which together may be the cause of orgasms experienced vaginally. Sexologists and other researchers are concerned that women may consider themselves to be dysfunctional if they do not experience G-spot stimulation, and emphasize that not experiencing it is normal. Two primary methods have been used to define and locate the G-spot as a sensitive area in the vagina: self-reported levels of arousal during stimulation, and stimulation of the G-spot leading to female ejaculation. Ultrasound technology has also been used to identify physiological differences between women, and changes to the G-spot region during sexual activity. The location of the G-spot is typically reported as being about 50 to 80 mm (2 to 3 in) inside the vagina, on the front wall. For some women, stimulating this area creates a more intense orgasm than clitoral stimulation. The G-spot area has been described as needing direct stimulation, such as two fingers pressed deeply into it. Attempting to stimulate the area through sexual penetration, especially in the missionary position, is difficult because of the particular angle of penetration required. Women usually need direct clitoral stimulation in order to orgasm, and G-spot stimulation may be best achieved by using both manual stimulation and vaginal penetration.