The fractional variation of the @R05326-1@ frequency of a @N04256@ in nuclear magnetic @R05326-1@ (NMR) @S05848@ in consequence of its magnetic environment. The chemical shift of a @N04256@, \(\delta \), is expressed as a ratio involving its frequency, \(\nu _{\mathrm{cpd}}\), relative to that of a standard, \(\nu _{\mathrm{ref}}\), and defined as: \[\delta = \frac{\nu _{\text{cpd}}- \nu _{\text{ref}}}{\nu _{ref}}\] \(\delta \)-values are normally expressed in \(\mathrm{ppm}\). For ¹H and ¹³C NMR the reference signal is usually that of tetramethylsilane (TMS), strictly speaking in @D01739@ in CDCl₃. Other references are used in the older literature and for other solvents, such as D₂O. @R05326-2@ lines to high frequency from the TMS signal have positive, and @R05326-1@ lines to low frequency from TMS have negative, \(\delta \)-values (arising from relative @S05644-1@ and @S05644-2@ respectively). The archaic terms '@D01856@' and '@U06575@' should no longer be used. For nuclei other than ¹H, chemical shifts are expressed either in the same manner relative to an agreed substance containing the relevant @N04256@ or relative to the ¹H @R05326-1@ of TMS as \(\mathit{\Xi}\) values, defined in the references below.