The cajun accordeon

A cajun accordeon is a one-row diatonic button accordeon. It is used for playing cajun music: this is an emblematic music of Louisiana and rooted in the ballads of the French-speaking Acadians of Canada. Cajun music is often mentioned in tandem with the creole-based, cajun-influenced zydeco form, both of Acadiana origin.

Accordeons reached Acadiana ( French Louisiana ) in the 1890s and became popular in the early 1900s. Due to World War II many factories that produced accordeons for the American market were destroyed. As a result some Cajuns began producing their own instruments, based on the one-row German accordeons, but with modifications to suit the nuances of the cajun playing style.
The cajun accordeon became a single-row diatonic accordeon, with multiple reeds for every button. The number of reeds sounding, is controlled by four stops or knobs. Mostly the cajun accordeon is tuned in the C key, but the D key is also relatively common. Some rare accordeons are constructed in the key of B flat.
Click the link to find some more information on the four stops
Click the link for a short introduction to the cajun accordeon

Louisiana constructed accordeons are usualy built in small backyard shops. The shop of Clarence "Junior" Martin of Lafayette Louisiana, who is a craftsman who builds beautiful instruments, is known for its extraordinary designs and tuning.
Though the instrument is called a cajun accordeon, both zydeco and creole musicians do play the cajun accordeon, each with their own sound.
picture from www.korbo.com
Sources: http://www.korbo.com/  https://en.wikipedia.org/wiki/Cajun_accordion  http://www.meloche.net/acchist.htm