The fractional variation of the @[email protected] frequency of a @[email protected] in nuclear magnetic @[email protected] (NMR) @[email protected] in consequence of its magnetic environment. The chemical shift of a @[email protected], \(\delta \), 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 @[email protected] in CDCl₃. Other references are used in the older literature and for other solvents, such as D₂O. @[email protected] lines to high frequency from the TMS signal have positive, and @[email protected] lines to low frequency from TMS have negative, \(\delta \)-values (arising from relative @[email protected] and @[email protected] respectively). The archaic terms '@[email protected]' and '@[email protected]' 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 @[email protected] or relative to the ¹H @[email protected] of TMS as \(\mathit{\Xi}\) values, defined in the references below.