Does exactly as it says on the tin: converts and cornet sized mouthpiece to fit in a trumpet mouthpiece receiver. I've tried it with a number of different cornet mouthpieces and several different trumpets, and have been impressed by every combination. Beware, however that it is very easy to get the mouthpiece stuck in the adapter if pushed in too forcefully. NOTE: DOES NOT FIT FLUGEL HORN RECEIVERS/MOUTHPIECES