Blocks of similar experimental units
Experiments with matched pairs or matched groups are special kinds of randomised block experiments. In them, a collection of experimental units is grouped based on similar weights, ages, etc. In other situations, the experimental units naturally separate into groups.
If the number of experimental units in each block is a multiple of the number of factor levels in the experiment, it is possible to randomly allocate each factor level the same number of times within each block. The resulting experiment is called a randomised block design.