The fractional variation of the resonance frequency of a nucleus in nuclear magnetic resonance (NMR) spectroscopy in consequence of its magnetic environment. The chemical shift of a nucleus, δ, is expressed as a ratio involving its frequency, ν cpd, relative to that of a standard, ν ref, and defined as: \[\delta = \frac{\nu _{\text{cpd}}- \nu _{\text{ref}}}{\nu _{ref}}\] δ-values are normally expressed in ppm. For ¹H and ¹³C NMR the reference signal is usually that of tetramethylsilane (TMS), strictly speaking in dilute solution in CDCl₃. Other references are used in the older literature and for other solvents, such as D₂O. Resonance lines to high frequency from the TMS signal have positive, and resonance lines to low frequency from TMS have negative, δ-values (arising from relative deshielding and shielding respectively). The archaic terms 'downfield' and 'upfield' 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 nucleus or relative to the ¹H resonance of TMS as Ξ values, defined in the references below.