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

What is a compensating euphonium?

Euphonium – Prodige

Trivia:The Compensating System for the Euphonium - Musical Instrument Guide - Yamaha Corporation

Bb Compensated Euphonium 'Dolce' - EP100 – Wessex Tubas

Jupiter 3+1 Valve Professional Compensating Euphonium - Silver-Plated

K.Custom Silver Compensating Euphonium - Artist Series Brass

Bb euphonium, compensating system, with soft case

Normans Musical Instruments Blog – Tagged Technical Guides

What Is a Compensating Tuba?

Bb Compensated Euphonium 'Dolce' - EP100 – Wessex Tubas