Brassworks
A trombone, at its most basic level, is simply a large bugle. Brass instruments all work in much the same way, yet the trombone is just a bit different. Brass instruments all work by taking the air from the player, combining it with the vibration from the player's buzzing lips, and amplifying it through a brass, silver or steel tube with a megaphone cone shape at the end. Originally, all horns were straight and played just a handful of notes. The introduction of tube bending gave brass instrument makers unparalleled freedom, as they could now hold 10 feet or more of brass tubing in a compact package. The trombone is able to vary the length of that tubing on the fly, giving it a range far beyond the old style straight tube, valveless horns of the past.
Making the Sound
To play the trombone, a player presses his lips to the mouthpiece and buzzes them while expelling air (like blowing a big raspberry). By altering the lip pressure, a player can alter the tone according to the trombone's own fundamental vibrations. Without using the slide, the trombone is capable of five or six notes (more for advanced players). The slide extends the range through many positions, each capable of five or six fundamental notes. In the Western system of music, this effectively extends the range of the instrument to over 40 notes (more for advanced players).
Design Variations
There have been a number of notable design variations and specializations which alter the way a trombone makes and alters sound. The addition of valves and extra tubing allows the player to reach lower slide positions quickly by adding length to the tubing. Instead of having to reach to the end of the slide's length for seventh position, for example, a player can just hit a valve to reach the notes, making quick transitions much easier.