The trombone emerged as a variation of the medieval Tumpet when the slide, in the form of a U bend, was created (mid 15th century), immediately producing an efficient and unique low brass instrument capable of playing all chromatics. From that time to the present, the instrument has consisted fundamentally of a bell section including attached inner slides, outer slides, and a moouthpiece, the tube being cylindrical up to a gradual expansion forward the bell.
The early trombone (sackbut) differs from its modern conterpart primarily by its marrow of heavier metal, its small expansion at the end, and its funnel shaped mouthpiece similar to the horn, the result being a mellow tone with command of soft through moderately loud dynamics, eminently suitable for combining with voices, soft woodwinds, or strings. It was soon characterized, and civic functions.
By the end of the 18th century, the trombone had acquired a flared bell and tubular stays. It had also lost popularity temporarily. It revival was sparked by the need of louder dynamics. Makers responded to this demand, thinning the metal, enlarging the bore, and changing the mouthpiece to a hemispherical shape with a sharp edge at the throat. Opera, having used trombone spradically, now explored their brilliant tone, and by the mid 19th century German orchestras had adopted three as a standard; the E-flat alto, B-flat tenor, and F bass.
The French, however, used three tenors. Eventually the French usage prevailed, and the alto become obsolete. The third tenor was combined, by means of a left hand thumb valve, with tubing to extend the horn to the bass range.