What is a Compensating Brass Instrument?

The term compensating is one that is often referred to but not always fully understood. It is normally used with reference to lower brass instruments (eg. Euphoniums & Tubas) which are either 'compensating' or 'non-compensating' models. There is a common misconception that if an instrument has a 4th valve* it is compensating, but that is not necessarily the case. I'll try to explain exactly what it means as clearly and concisely as possible, although that's easier said than done!
The term compensating is one that is often referred to but not always fully understood. It is normally used with reference to lower brass instruments

Tuba/Euphonium — Dana Hofer Brass Instrument Repair, Inc

John Packer JP274 Compensating Euphonium

Low Brass, Amro Music

Saw this on , compensating 4th valve? : r/trumpet

Mack Brass EU1150 3+1 Valve Compensating Euphonium - musical

Bb-Euphonium Brass with compensating valve Incl. case with back pack set and mouthpiece.

Monzani MZEP-1150L Bb-Euphonium Brass, Compensating Valve

Brass Euphonium Bb Euphonium 3+1 Pistons Compensating System with

Willson 2900-TA-1 Compensating Euphonium

Bb Compensated Euphonium 'Dolce' - EP100 – Wessex Tubas

Immaculate compensating euphonium For Fascinating Sound Notes

Euphonium – Prodige