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

Besson Compensating Euphonium

Brass Euphonium Bb Euphonium 3+1 Pistons Compensating System with

Here is a BRAND NEW John Packer JP374 'Sterling' model compensating euphonium. The JP374 Sterling is JP's flagship 4 valve fully compensating

John Packer JP374 'Sterling' Compensating Euphonium in Silver Plate SN 37411063 BRAND NEW

Besson Imperial Four Valve Compensating Euphonium GREAT PLAYER

Trivia:The Compensating System for the Euphonium - Musical

7 Brass Instruments: Differences in sound & playing style – t.blog

Knopf Compensating Double Horn (no serial#) DD - Pope Horns Inc.

Normans Musical Instruments Blog – Tagged Technical Guides

Compensating Valve Systems on Brass Instruments

Euphonium – Prodige