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

Bb euphonium, compensating system, with soft case » Stagg

Yamaha Model YEP-842TS Custom Compensating Euphonium MINT

What is the Compensating Valve?

Band & Orchestra » Brass Instruments » Euphoniums » Stagg

Hampson Horns - Compensating doubles are a great option

This old Boosey - Keith Polito Brass Instrument Repair

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

Image is indicative only, Take the next step in your brass playing with the Cambridge Classic Compensating Euphonium., The Cambridge Classic series

Cambridge Classic 4 Valve Compensating Euphonium

What is the Compensating Valve?

John Packer JP274 Compensating Euphonium

John Packer JP374ST Euphonium – Musical Sales Online

Brass Euphonium Bb Euphonium 3+1 Pistons Compensating System with

Paxman - Professional Model 33 Bb/F (A-+) Compensating Double