The  following is a light re-write of an article that I originally wrote for Wikipedia  ~ Fañch

Treujenn Gaol (Breton: cabbage stalk) is the Breton term for an older-style clarinet with Albert or Mueller system keywork as used in Breton music. The term 'treujenn gaol' was originally a pejorative term invented by bombard players who found the newer instrument encroaching on their livelihood. Their use is concentrated in central western Brittany - Bro Fañch and Plinn and Bro Fisel. Similar to the bombard/biniou pairing, clarinets are frequently associated with the accordion in call-and-response pairings, although pairs of clarinets are also common. 

The clarinet arrived in Brittany in the 19th century. The most traditional Breton clarinet is an older type of instrument with 13 or even fewer keys, in contrast to the modern 'Boehm' instrument commonly used in most contemporary music. These instruments typically employ some variation of the Müller keywork system. Classical musicians in the 19th century discarded older instruments in favor of  the Boehm system, replacing the formerly ubiquitous 13-key clarinet in the 19th century. These discarded instruments eventually found their way into the hands of folk musicians and the 'treujenn gaol' was born.

After a decline in use in traditional music in the early 20th century, the instrument was revived in the 1970s for use in pairs of instruments (much like the bombard and biniou) and in the music of the bagad. In the current day, modern Boehm-system clarinets are widely used, but tend to be referred to as clarinettes.

The treujenn gaol style is mostly concentrated in the clarion register and will play within an octave range.  Earlier players would sometimes remove the register key, limiting the range of the instrument to the clarion range and above. The organization Paotred an Dreujenn Gaol has been particularly active in promoting research, documentation, and performance of this instrument.

A few well-known Breton clarinetists: Christian Duro, Dominique Jouve, Emilien Robic, Jil Lehart, and Erik Marchand. The bands Termajik, Fleuves, Darhaou,  and Imperial Gavotte Club are just a few that feature the clarinet.