Fisher inequality

A constraint on the parameters of a balanced incomplete block design. A BIBD is an incidence system on a set $V$ of $v$ points and a family $B$ of $b$ blocks: subsets of points, each of size $k$. It is required that each point be contained in just $r$ blocks and each pair of points in just $\lambda$ blocks.
Fisher's inequality states that $$b \ge v \ .$$
A symmetric design is one in which equality holds: $b = v$.