In a nuclear magnetic resonance spectrum of the type AMX, the relative signs of the spin coupling constantsJAMandJMXcannot be determined by double resonance experiments ifJAXis unobservably small or zero, but the problem can be solved by the simultaneous application of three radio-frequency fields. This technique is described and demonstrated, and it is suggested that it should prove particularly useful for relating the signs of long-range spin coupling constants to couplings of known sign. Applied to the molecule of 3-bromothiophene-2-aldehyde, this experiment establishes that the long-range coupling to the aldehyde proton has the same sign as the vicinal coupling.