The keys on an instrument are made up from several components. Whilst you don't normally need to know this it is helpful if you decide to have a go at doing your own maintenance. At the very least it will give you a clearer insight as to what problems might occur over time, and will certainly help in understanding some of the other pages on this site!

Here's a typical key, with its component parts labeled.

Keys come in a wide variety of designs, shapes and sizes - some have cups on them, others are merely linking arms. The one thing they all have in common is that they pivot - and are therefore prone to wear.

From top to bottom:

A compound key, with one key missing showing the rod that runs internally through the group.
A ring key.
A link key ( no cup ), in this case a saxophone octave key touchpiece.

Compound keys are worth an extra line or two.

Most keys act as single entities - you press them, they do their thing. Compound keys are linked - and whilst they can act individually they can also act as a group...though typically in pairs. That's to say that when you press one key down, it brings another key down with it.

The right and left hand action on saxophones ( known as the 'stack' keys ) can do this too, though as the keys are not secured to their pivoting rod they're not true compound keys.

Keys are generally made of nickel silver or brass ( for saxes ), though it's not uncommon to find expensive instruments with solid silver keywork. This makes them eminently repairable in the event of damage or wear, or even customizable. The one exception is those keys made from a die-cast metal known as 'Mazac'. Some early 70's Boosey & Hawkes clarinets were fitted with this type of key, and when they break they're impossible to repair due to the extremely low melting point of the Mazac.

The keys as a whole form what's know as 'the action'.