At one time in the 19th century trombones had valves like other brass instruments. But that style didn’t last.The trombone sounds as written which is different from other brass instruments. This is true because it sounds the note as written in the music. It is the brass instrument that uses the glissando the most. It is a long brass tube that is folded back on itself sort of like a paper clip. The mouthpiece of a trombone is pretty large and is cup-shaped. You play the trombone by sliding tubing back and forth to make the tube longer or shorter. This changes the sound.
A trombone has seven positions. To make other pitches the player changes his lip shape. When he squeezes or loosens his lips this changes the sound. A trombone can also do a glissando, which is where the player goes up to a note or down from a note, playing all the notes in between by just moving the slide.
The trombone is the only modern orchestral brass instrument that could play all the notes of the chromatic scale from the beginning. But they weren’t in early orchestras. Composers thought they were only for solemn music. When composers write music for the trombone they usually write it for three trombones to play at a time and they usually play the harmony.