Another type of listing involves identifying pairs of combinations: the handshake problem.

This shows how pairs of items interact. Draw up a table, with the set of items listed as both rows and as columns.

Check the type of interaction. For example, a handshake with another person is a joint interaction: Bob shaking hands with Alice is the same as Alice shaking hands with Bob. However, if Alice and Bob exchanged gifts, then both directions (Alice to Bob and Bob to Alice) need to be identified.

Sometimes items may interact with themselves (a domino can show a pair of 1s), and sometimes they may not (Bob will not shake hands with himself).