Brass instruments are made of, er, brass. But you could just as well say that brass instruments are made of air-a volume of air that is contained within the long metal tube. When it's made to vibrate, this volume of air will produce a note. You can get the effect by blowing over the neck of a bottle, which makes the air inside vibrate. Now put some water in the bottle and blow again. The note will be different (higher or lower?) because the length of air in the bottle is reduced.

Brass musicians play into their instruments, of course, not across them, but the principle is the same. And they don't normally pour water into their instruments. On the contrary, they occasionally have to let the water out that condenses inside the instrument from their breath. Instead they have other ways to change the note.

One way is by changing the shape of their mouths (there is a French word for this mouth shape: 'embouchure') as they blow. A certain length of air, contained in a bottle or tube, will have a certain lowest note it can produce. This is known as the 'fundamental', and it comes from having the air vibrate at its lowest possible speed. But the air can also be made-by the player's embouchure in the case of brass instruments-to vibrate at twice that speed, three times, four times, and so on. Each speed of vibration, or 'frequency', will be heard as a new note.

The oldest kind of brass instrument is the trumpet. As already suggested, they don't have to be made of brass: they could be animal horns or conch shells-anything to contain a volume of air. Metal trumpets were being made in Egypt three and a half thousand years ago. People probably liked the special brightness that came from having the metal vibrate along with the air. They still do.

Lower in pitch are the trombone and the horn. The trombone can change the length of its volume of air by sliding, and this will change the note. Like the trumpet, the trombone is basically a straight tube, flaring at the end. The horn, on the other hand, is more of a long thin cone-or would be, if you could unwind it. Lower still, because bigger, is the tuba, which is the bass instrument of the brass section.

Where the trombone has its slide, the trumpet, the horn and the tuba all have keys, which the player can push down to change the length of tube in play, thereby change the length of air vibrating, and thereby change the note. This feature was introduced in the early nineteenth century. Before that, trumpets and horns could play only a limited range of notes.