An adapter for the finger appendage of a brass instrument has an adapter body. An outboard side and an inboard side of the adapter body are spaced apart along a width. The adapter includes a fitting portion defining the outboard side of the adapter body and a spacer portion defining the inboard side of the adapter body. The fitting portion defines a mounting opening configured to receive the first portion of the adapter body to mount the adapter body on the finger appendage such that the spacer portion is received in the finger gap and the inboard side of the adapter body opposes the second portion of the finger appendage to define a reduced gap having a width that is less than the width of the finger gap.