What is the fourth valve for

The fourth valve does two jobs, and both matter.

It extends the range downward. On its own the fourth valve lowers the instrument by a perfect fourth, opening up the notes below the normal three-valve bottom and reaching down toward the pedal register. Without it, the low B natural is unreachable by normal fingering.

It fixes intonation. The standard three-valve combinations for the lowest notes — 1+3 and 1+2+3 — are acoustically sharp. Substituting the fourth valve (4 for 1+3, 2+4 for 1+2+3) gives a much better-in-tune alternative, because a single longer valve loop is closer to the correct length than several short ones stacked together. This is the same problem the compensating system attacks from the other direction.

So a working euphonium player learns fourth-valve and combined fingerings not as optional extras but as the default way to play the low register in tune. If your instrument is non-compensating, these alternate fingerings are your main intonation tool; if it’s compensating, they still help and the two systems reinforce each other.

This is one question under Technique — the full treatment lives there.

• fourth valve
• fingerings
• technique
• range