Choosing a Trombone
Choosing by bore size

The diameter of a tube is referred to as its "bore." The size of the bore changes the amount of breath required to play as well as the resistance of the instrument-which also affects the timbre. Trombone bores generally come in three sizes: small bore, medium bore, and large bore. The smaller the bore, the less breath is required, resulting in an instrument that is easier to play and which boasts a subtle, mellow timbre. A larger bore requires more breath and produces a rich, flamboyant timbre and a higher volume. There are also instruments which feature inner slides of two different sizes. These are called dual bore trombones.

Bore Characteristics
Large bore These instruments are loud and have a timbre well-suited to harmonizing when playing in an ensemble. They are favored by trombone players in large orchestras and are often used nowadays for playing wind music.
Medium bore These instruments are comparatively new and fall somewhere in between the large and small bore instruments. They are well-suited for the first chair in an orchestra, or for wind music that requires a fuller sound.
Small bore These instruments produce an overall bright sound, and are adept at playing delicate high notes. They are better suited for solo performances, and are favored by jazz and pop music players.
Dual bore There are instruments which feature inner slides of two different sizes. The lower slide is slightly larger than the upper slide. This affects the timbre and sound in subtle ways, such as producing a wider and softer sound.