A method of determining the relative signs ofJABandJBXin nuclear spin systems of the type AB2Xqis described and applied to the AB2X3proton spin system of 2-butyn-1-ol. Variation of the relative chemical shiftvA—vB(through changes in temperature or in the composition of the solvent) causes certain pairs of ‘labile’ transitions in the X spectrum to coalesce on one side or other of the X spectrum. The information about the relative signs of the coupling constants is obtained by noting the two different critical values ofvA-vBat which these coalescences occur. Since the method remains applicable even if the third coupling constantJAXis vanishingly small, it is particularly useful in the determination of the signs of long-range spin coupling constants where the usual double resonance methods often break down. It is concluded that the long-range couplingJ(H-C-C≡C-C-H) of 2-butyn-1-ol has a positive sign.