A casement espag (or espagnolette) is a type of locking device used in casement windows and doors. It is a metal strip that runs vertically along the sash of a casement window and is attached to the handle. When the handle is turned, the espag engages a series of locking points along the frame, securing the window in place.

Overall, the casement espag is a reliable and secure locking system that is commonly used in modern casement windows and doors.