MOD 2 PROBLEMS
These are very elementary problems concerning legal positions that would no longer be legal if the colours and the board were reversed, and the other player had the move. The array at the beginning of the game is a good example of such a position.
This is a fairly well-trampled field with limited scope for novelty, but there is still some space for original ideas. They are fun, but I feel a bit guilty for enjoying something so simple-minded. For 30 or so other compositions, see the Chess Problem Database Server and search for k="Thema Parit%t". [Sic: "%" denotes a wildcard, if you can't type "ä".]