A useful application of long range couplings for the assignment of ring-fusion stereochemistry in decalin ring system bearing an angular methyl group has been developed (Williamson, K. L.; Howell, T.; Spencer, T. A. J. Am. Chem. Soc. 1966, 88, 325). In the trans-decalins, there are usually several ideal W-pathways for long range coupling between the methyl group and axial protons. In cis-decalins, there are fewer or no such pathways. Thus in a pair of cis/trans isomers, the methyl group in the trans isomer will usually be broader (or will actually show splitting), whereas the cis isomer will have a sharper (unsplit) methyl group.
