When African slaves were taken to America they had nothing with them except their spirit, fortitude and music. They would make all kinds of instruments to recreate the sounds of their homeland including primitive types of banjo. In the mid 1800s guitars were far too expensive for the average working man and so they made their own using whatever materials they could lay their hands on. Cigar boxes made good sound boxes. Add a neck and strings and lots of ingenuity and you had the first CBG.
Traditionally the CBG had three strings and was tuned to an open chord which made it fairly easy to play. Played with a slide made of bone or later steel tube the sound produced became the sound of the blues.
When you pick up a CBG for the first time you will be amazed at how easy it is to get that swampy sound associated with the Delta. With only three strings there are less notes available than on an ordinary guitar and therefore you must by necessity become more creative and also a more interesting player.