This is a JK small shank mouthpiece. Inner diameter: 26.00mm, medium (from the JK website) JK seal. Comes with a mouthpiece brush. I ordered this for trial purposes about two years ago, but the other size suited me better, so I'm selling it. It's new, but there are some shank scratches from the trial, so I'm listing it as "near new." Suitable for baritone horn, small shank euphonium, tenor trombone, or with an adapter, for a medium-small shank euphonium.