170:1 There are two "corners" ("payos") on the head, at the place on either side where the jaw joins near the ears. Some authorities prohibit even to cut them with scissors like a razor, i.e., very close without leaving anything of the hair near the skin. Therefore, if one needs to shave this area for a medical reason, he should be careful not to shave completely to the skin. The length of the payos is across from the hair on the temples to below the ear, where the bottom of the cheek meets the jaw.
170:2 The Torah only prohibits using a razor to cut the corners of the beard. There are five such corners and many positions when it comes to the details, so a pious person should not use a razor on any part of his beard, nor even on his mustache or on his neck. There is no difference between an actual razor and a sharp stone that cuts hair: they are equally prohibited. Those who remove their beard using a depilatory cream must be careful not to scrape the cream off with a knife, as that might cut the hair. Instead, they should use something like a wooden chip.