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

XO 1270 Professional Series Compensating Euphonium

Compensating Valve System - Charlie's Tuba Site

Available in gold lacquer or silver plate with gold trim., The Cambridge Classic non-compensating 4 valve Baritone Horn is the perfect step-up for 3

Cambridge Classic 4 Valve Non-Compensating Baritone Horn

John Packer JP374ST Euphonium – Musical Sales Online

Euphonium - Understanding the Compensating System - Geoff Durbin

Normans Musical Instruments Blog – Tagged Technical Guides

Trivia:The Compensating System for the Euphonium - Musical

1270 XO Euphonium

Willson 2900-TA-1 Compensating Euphonium

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