A Butterfly joint is a type of joint used either to hold two or more wooden boards together or to keep two halves of a board that have already started to split from splitting further. They may also be used to stabilize the core of a knothole, preventing it from dropping out over time. They are also commonly used as decorative inlays for non structural aesthetic purposes.
A negative of the hole is cut out of the board the butterfly will be placed in and the butterfly is then fitted, keeping the joint together. The wood used for the butterfly is usually a contrasting wood but not always.
Butterfly joint have been used both in decorative and structural joints since ancient times. They were prominently used in construction of the Cairo Dahshur Boats, a type of Khufu shop from the Egyptian middle kingdom. They were also historically used in repairing cracks in dutch tabletops in the 18th century. This is where the other name of this type of joint, Dutchman joint, originates. The butterfly joint was installed across the crack to stabilize and inhibit further movement of the crack.
Another name is Dovetail key as it looks like two Dovetails opposed by their tips.