Invented by Ken Titmus at Warburton USA, this handy little tool measures the distance between the end of your mouthpiece shank and the beginning of the lead pipe in your trumpet receiver - known as the "GAP".
Did you ever wonder why a given mouthpiece works well in one trumpet, but not very well in another? Often times this problem can be traced to the GAP being too large or small.
In a perfect world, all trumpet manufacturers would get the sizing of the mouthpiece receiver and the position where the lead pipe is soldered into the receiver exactly the same - and trumpet mouthpiece receivers would never wear - and mouthpiece shanks would all be exactly the same size taper. In reality, these perfect conditions don't exist - which in turn results in variations in the amount of GAP.
To complicate things a little more, there is no single "correct" GAP size. The amount of GAP can and will vary between great playing mouthpiece/trumpet combinations.
Too little or too much GAP causes playability problems which can be very difficult to overcome.