Bore size refers to the internal diameter of the slide tubing, and it is one of the main specs that separates trombones built for different musical settings, independent of whether the horn has an F-attachment.
- Small bore (roughly .481"-.500" / around 12.2-12.7mm) trombones are lighter and easier to move air through at speed, typically favored for jazz and commercial playing where agility and a brighter, more focused tone matter.
- Large bore (roughly .547" and up / around 13.9mm+) trombones need more air to fill but produce a bigger, darker, more resonant sound, and are the standard for orchestral and concert band settings.
- A common student/school instrument is a medium-large or large bore trombone, since most school band and orchestra repertoire calls for that sound - this is worth checking against what the instrument is actually being bought for.
Bore size is stamped or listed in a model's specifications and is worth confirming rather than assuming, since two trombones that look similar can have meaningfully different bores - and a mismatch between bore size and the music actually being played is a common source of buyer disappointment after purchase.
