relays as the common contact element provides a mechanically economical method of providing a higher contact count. Form D contacts Form D contacts ("continuity transfer" contacts) differ from Form C in only one regard, the make-break order during transition. Where Form C guarantees that, briefly, both connections are open, Form D guarantees that, briefly, all three terminals will be connected. This is a relatively uncommon configuration. Form K contacts Form K contacts (center-off) differ from Form C in that there is a center-off or normally-open position where neither connection is made. SPDT toggle switches with a center off position are common, but relays with this configuration are relatively rare. Form X contacts Form X or double-make contacts are equivalent to two Form A contacts in series, mechanically linked and operated by a single actuator, and can also be described as SPST-NO contacts. These are commonly found in contactors and in toggle switches designed to handle high power inductive loads. Form Y contacts Form Y or double-break contacts are equivalent to two Form B contacts in series, mechanically linked and operated by a single actuator, and can also be described as SPST-NC contacts. Form Z contacts Form Z or double-make double-break contacts are comparable to Form C contacts, but they almost always have four external connections, two for the normally open path and two for the normally closed path. As with forms X and Y, both current paths involve two contacts in series, mechanically linked and operated by a single actuator. Again, this is also described as an SPDT contact. Make break order Where a switch contains both normally open (NO) and normally closed (NC) contacts, the order in which they make and break may be significant. In most cases, the rule is break-before-make or B-B-M; that is, the NO and NC contacts are never simultaneously closed during the transition between states. This is not always the case, Form C contacts follow this rule, while the otherwise equivalent Form D contacts follow the opposite rule, make before break. The less common configuration, when the NO and NC contacts are simultaneously closed during the transition, is make-before-break or M-B-B. Electrical ratings Contacts are rated for the current carrying capacity while closed and the voltage breaking capacity when opening (due to arcing) or while open. Opening voltage rating may be an A.C. voltage rating, D.C. voltage rating or both. Arc snuffing When relay