In a BDSM relationship the partner who has the active (i.e. controlling) role in a session or in the entire relationship is described as the "top", a role that often involves inflicting pain, degradation or subjugation. The partner referred to as the "bottom", or more frequently as the sub (short for submissive), submits voluntarily to the actions of the top. Although the top usually is the dominant and the bottom the submissive partner, it is not inevitably so. In some cases the top follows instructions, i.e., he "tops" the bottom according to the bottom's desires and in a way the bottom expressly requires. A top only having apparent control, while in reality conforming to the instructions given by the bottom, is referred to as a service top. In contrast a dominant top controls their submissive partner by using physical or psychological techniques.
A similar distinction also may apply to bottoms. At one end of the spectrum are those who are indifferent to, or even reject physical stimulation. At the other end of the spectrum are bottoms who enjoy physical and psychological stimulation but are not willing to be subordinate to the person who applies it. The bottom works with the top specifies the basic conditions of the session and gives instructions, directly or indirectly, in the prelude to the session, while the top respects this guidance.
A switch is someone who participates in BDSM activities sometimes as a top and other times as a bottom or (in the case of dominance and submission) sometimes as a dominant and other times as a submissive. Partners may switch roles based on mood, desire, or to allow each partner to experience their preferred activity. For example, a switch may be in a relationship with someone of the same primary BDSM orientation (e.g., two dominants), so switching provides each partner with an opportunity to realize his or her BDSM needs. It is also common for people to switch with different partners, such as when a person acts exclusively as a top with one partner and exclusively as a bottom with another.