The gluteal sulcus (also known as the gluteal fold or horizontal gluteal crease; sulcus is Latin for furrow or ditch) is the slight fold where the buttocks meet the legs. It is more pronounced when a person stands or lies with their legs straight, and it is also more pronounced on some people than on others. This will depend on gender, general body shape and muscle tone.
Generally it is best not to administer corporal punishment below the gluteal sulcus, although that is a matter of personal preference of the giver and the recipient. The area near the gluteal sulcus is known as the "sweet spot" in spanking, it is also the part where a person's weight goes when sitting upright.