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Reference.org
Reference designator
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<p> A reference designator unambiguously identifies the location of a <a href="/facts/Electronic_component/F3NBVv4Q">component</a> within an <a href="/facts/Circuit_diagram/Vqk3udN0">electrical schematic</a> or on a <a href="/facts/Printed_circuit_board/muSR2p43">printed circuit board</a>. The reference designator usually consists of one or two letters followed by a number, e.g. C3, D1, R4, U15. The number is sometimes followed by a letter, indicating that components are grouped or matched with each other, e.g. R17A, R17B. The <a href="/facts/IEEE/UddC4pUW">IEEE</a> 315 standard contains 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 for capacitors, K for relays. </p><p>Industrial electrical installations often use reference designators according to <a href="/facts/IEC_81346/I0mnEdqk">IEC 81346</a>. </p>