The Structure of the Trombone
[Experiment]I can only extend my right arm so far!

Each trombone has a maximum slide position it can be played at. For a tenor trombone this is around 60 cm. This is right around the maximum distance that the average Japanese adult male can extend his arm.
However, what if your arms are simply not long enough? One clever way of overcoming this is to attach a string to the slide, allowing you to reach the farthest slide position.
The reason you extend your arm, in the end, is to extend the length of the tube. Trombone manufacturers then came up with an idea. Why not attach another piece of tubing somewhere on the instrument? Doing so would allow the player's breath to pass through this new piece only when needed.
This additional tubing also plays a role in extending the bass range of the trombone. For tenor trombones, this extends the range from B♭ (their fundamental pitch) to A, G, and F. The additional tubing is thus called an "F attachment." A valve must also be installed to use this additional tubing.
This has made it possible for even smaller people to play the trombone.

The valve makes it easier to play low notes without having to extend your arm as much, and can even be switched out to change the sound completely.
There are even bass trombones with two valves. Turning on and off the two valves gives a total of four combinations, for four different sounds.
Trombone valves were first invented in the 19th century. Until then, it was extremely difficult to play continuously from, say, B♭ to C. Valves now make this easy.