Weak alignment of dissolved protein molecules in a magnetic field leads to the incomplete averaging of dipolar interactions between pairs of atoms, represented by the numbered vectors. These unaveraged interactions appear in NMR spectra as residual dipolar couplings (RDCs) — splitting of peaks in the spectrum that scale proportionally with the alignment strength, and that depend on the individual projections of dipolar interaction vectors onto the direction of maximum molecular alignment. a, In static molecules, RDC measurements of covalently bonded pairs of chemically identical nuclei depend only on the angle between the direction of maximum alignment and the specific bond. b, For dynamic molecules, submillisecond molecular motions introduce local disorder, which appears as an apparent reduction in alignment strength, as indicated by the shortened vectors. The resulting convolution of orientational and dynamic contributions to the RDC measurements can be untangled by performing several experiments in different solvents that cause the molecules to align in different ways. Salmon et al.1 use this deconvolution approach in their method for analysing protein dynamics.