The two relata can be paired in four ways:
1. sensory source and sensory target;
2. conceptual source and conceptual target;
3. sensory source and conceptual target;
4. conceptual source and sensory target.
When both terms are sensory, the common feature may itself be sensory or conceptual, and the first configuration subdivides further. In the book's finer accounting, the sensory–sensory branch has three forms, while each of the other three branches has one. This gives six types in all.
The count belongs to the author's specific analysis of the common feature and should not be treated as an unrestricted combinatorial exercise.