Most techniques for determining focal mechanisms attempt to find a unique focal mechanism. However, for the usual case in which only a limited number of P–wave first motions are available, the focal mechanism can only be limited to a domain in which any solution will be equally valid. If the data contain incorrect readings then only domains with a limited number of errors can be found. A computer program to facilitate the defining of domains in which the P, T or B axis define valid focal mechanisms is presented and evaluated. The shape of the domain gives insight into the influence of recording station locations on the focal mechanisms. P–wave amplitudes or S–wave polarization, if available, could be used to indicate a preferred focal mechanism within the domain of valid solutions.