|The Annulus, or "Gap" as it is commonly referred to, is the space (chamber) between the end of a trumpet mouthpiece and the beginning of the leadpipe.
THE GAP IS THE MOST IMPORTANT DIMENSION
THAT CAN EASILY BE ADJUSTED TO:
|The "gap" affects the pitch center, or slot of every note on the trumpet.|
It does this by affecting the intonation of every note.
When the gap is not correct, you must compensate using your chops and air in an effort to make all the notes be in tune and sound as even in possible. This means you are working harder than you need to to play the instrument.
Signs of an incorrect gap:
|The video below explains what the gap between the mouthpiece and leadpipe is.