A reference designator unambiguously identifies a component in an electrical schematic or on a printed circuit board and usually consists of one or two letters followed by a number, for example R10. The number is sometimes followed by a letter, indicating that components are grouped or matched with each other, for example, R12A, R12B. The Institute of Electrical and Electronics Engineers (IEEE) has a standard with a list of class designation letters to use for electrical and electronic assemblies. For example, the letter R is a reference prefix for the resistors of an assembly, C represents capacitors, and K represents relays.
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