Block design

Summary

A block design is a finite set of points together with a family of subsets of the same size, the blocks. It is balanced: every set of $t$ points lies in the same number of blocks. The most studied case is $t = 2$, the balanced incomplete block design. Block designs are a central kind of combinatorial design. They appear in the planning of experiments, in finite geometry and in coding theory. The node block design gives the precise definition.