Restless Knights

In the Dovetail Mate, the mating queen is one square diagonally from the mated king. She controls six of the nine squares needed for a checkmate (the eight squares around the king and the square he stands on make up the nine). She cannot protect the two squares that are a knight's jump away from her (here a rook and a bishop), so these two are occupied by friends of the mated king, getting in his way and blocking his escape. The queen doesn't control her own square, so this one needs to be protected by one of her friends, doesn't matter which one (here a rook). The similarity between the Dovetail and the Swallow's Tail is that in both cases the blocking pieces can't be knighs for the mate to work.