Activities and relationships in BDSM are often characterized by the participants' taking on roles that are complementary and involve inequality of power; thus, the idea of informed consent of both the partners is essential. The terms "submissive" and "dominant" are often used to distinguish these roles: the dominant partner ("dom") takes psychological control over the submissive ("sub"). The terms "top" and "bottom" are also used; the top is the instigator of an action while the bottom is the receiver of the action. The two sets of terms are subtly different: for example, someone may choose to act as bottom to another person, for example, by being whipped, purely recreationally, without any implication of being psychologically dominated, and submissives may be ordered to massage their dominant partners. Although the bottom carries out the action and the top receives it, they have not necessarily switched roles.