Keys

Although technically not valves, keys have the same purpose: to alter the length of the active air column inside the instrument.

Before the invention of the valves, instrument makers looked to the woodwind family for inspiration. They drilled large tone holes directly into the brass tubing and covered them with large, padded metal keys.
Here is how this alternative finger-key system controls the music:

1. The Open Air Column: When all the keys are held closed, the musician's breath travels uninterrupted all the way to the very end of the instrument, creating its lowest, foundational notes.
2. Venting the Tube: When a musician presses a key, it lifts a padded cup off a tone hole. This instantly creates an "escape hatch" for the sound waves, venting the air column early.
3. The Resulting Pitch: By escaping through the open hole, the air column is effectively cut short. Because a shorter tube creates a higher frequency, the pitch of the instrument instantly jumps upward.

An Evolutionary Dead End: While keyed bugles and the giant, upright Ophicleide were wildly popular in 19th-century military bands, they had a major flaw. Uncovering holes caused the air to lose pressure, making some notes sound muffled or fuzzy. When the precise, pressurized rotary and piston valves were invented, keys on brass instruments quickly became a relic of the past.