Encyclopaedia of DesignTheory: A tight single-change covering design

The array

1 1 1 1 1 3 3 3 2 2

2 2 5 6 7 7 7 7 7 6

3 4 4 4 4 4 6 5 5 5

is a tight single-change covering design as

each pair of its entries occurs in at least one column (block), so all pairs are "covered";
when each block is formed from the previous one, only a single change is made;
when each new block is formed, the new entry occurs only with entries with which it has not previously been paired, so the arrangement is "tight".