C Extensions give the low string on a double bass, an extra 4 low notes down to a C. They can be simply a fingerboard that is fingered by hand, or have mechanical locking latches.

My style of C extension is usually designed to highlight the original scroll, not to blend with it. I don't use screws to attach the extension and I don't cut into the scroll. I use black delrin for the latches as it is not affected by changes in humidity. Each latch is individually adjustable for string height, intonation and tension.