A framework is "just rigid" if it is rigid, but ceases to be so when any single bar is removed. Lamb (1928, pp. 93-94) proved
that a necessary (but not sufficient)
condition that a graph be just rigid is that
where is the number of edges (bars) and is the node of vertices (i.e., pivots;
Coxeter and Greitzer 1967, p. 56).
is the number of edges (bars) and 
is the node of vertices (i.e., pivots;
Coxeter and Greitzer 1967, p. 56).
