A symmetry extension of Maxwell's rule for rigidity of frames is found. Thi
s rule subsumes and strengthens Maxwell's 1864 rule by requiring that the i
nternal and external degrees of freedom of a pin-jointed structure are not
only numerically equal, but are also equisymmetric, A number of special cas
es of Maxwell's original rule are studied to show the improved insight that
the symmetry-adapted version can give. (C) 1999 Elsevier Science Ltd. All
rights reserved.