Many mammals (eg. dogs, cats, pigs) have a line of nipples along each side of their chest and abdomen from which they can feed milk to their young. In humans there is normally only one nipple on each side of the chest. Nipples in all mammals can actually develop at any point along a nipple (or milk) line. In humans it extends along the trunk from just below the center of the collar-bone (clavicle) to the top of the pelvis. They occur in about one in every twenty people of both sexes.
Accessory nipple are harmless and are often mistaken as moles, and only after excision is the correct diagnosis made. They can vary in size from merely an abnormal patch of hair to a fully functioning (but usually smaller) breast (pseudomamma).
In medieval times a significant accessory nipple was considered to be the sign of a witch as the devil could be fed from it.