The trombone is one of the oldest instruments in the orchestra. Among brasses, the only older ones are the trumpet and horn. In the Middle Ages, trumpets and horns, along with a reed instrument called the shawm, were used as signaling instruments by night watchmen on town gates. The same instruments were also used in battle both to signal to the warriors and to try to unnerve or scare the enemy. Trumpets were long (4′-8′), straight tubes, although they were usually made of of shorter tubes that could be taken apart for easier travel or repair and then be assembled when it was time to play. Valves would not be invented for hundreds of years, and trumpeters at the time did not know how to play any but the four or so lowest overtones.
As long as these instruments were used only as signaling instruments or noise makers, it did not matter if the trumpets could play only a few notes. But by the mid 1300s, groups of trumpets and shawms were also expected to provide music for dancing. The cumbersome length of the trumpet and limited number of notes it could play became problems. First, trumpet makers discovered how to bend short sections of tubing so that trumpets could be assembled in an S-shape or a loop, which solved the problem of length. Then someone discovered how to insert one tube into another. When a mouth pipe could be inserted into the rest of a trumpet, so the larger part could slide along it, the trombone was born (ca. 1400, give or take a decade.) We would call it a slide trumpet today, but an Italian document written in 1439 called it "trombone" (Italian for big trumpet).
By the time this new instrument became widespread, not only did towns still sponsor bands of shawms and trombones, but kings and dukes and others of the nobility did, too. These bands gradually stopped serving as night watchmen and started performing from towers and balconies for short daytime concerts. They still played dance music, but also took on new ceremonial duties, such as participating in processions.
Some time between about 1450 and 1490, someone discovered that linking two outer tubes with a bent tube had important advantages. Instead of having to move the heavy part over the light part, the player could move a comparatively light U-shaped slide over two inner tubes while holding the heavier part stationary. That is the basic principle of the slide trombone to this day. Another advantage is that the player could move the slide half as far to get the same amount of change in the sounding length of the whole instrument. It was much easier to play than the old design, but it took more than 200 years for the old slide trumpet to disappear entirely.
Not long after 1500, the trombone took on two new roles in addition to what it had been doing for a hundred years. It began to participate in church services, still as part of a band of trombones and shawms (or increasingly, trombones and cornetts). It also began taking part in expensive theatrical entertainments offered by powerful rulers (as well as weak rulers who wanted to appear strong) on politically important occasions such as dynastic marriages, baptisms of heirs, or state visits by other rulers.
These entertainments offered a break with the past and suggested new ways to use the trombone. No longer was it heard only in conjunction with shawms or cornetts. Composers experimented with groups of trombones only or with pairing a trombone with soft instruments or even to accompany singers. By 1590, it would have been easy to predict a great new future for the trombone, but tastes and economic conditions change. By 1630, it was starting to disappear.
