Crémone or "Crémone Bolt" is a type of decorative hardware used as a locking device

Lock (device)

A lock is a mechanical or electronic fastening device that is released by a physical object or secret information , or combination of more than one of these....

 to fasten a pair of swinging windows or "French doors". A knob or lever handle is linked by a rack and pinion gear

Rack and pinion

A rack and pinion is a type of linear actuator that comprises a pair of gears which convert rotational motion into linear motion. A circular gear called "the pinion" engages teeth on a linear "gear" bar called "the rack"; rotational motion applied to the pinion causes the rack to move, thereby...

 to a pair of half-round rods or "surface bolts" which slide over the outside surface of the door or window and extend into sockets

Socket

Socket may refer to:In mechanics:* Socket wrench, a type of wrench that uses separate, removable sockets to fit different sizes of nuts and bolts...

 at the head and sill of the opening. The gear mechanism operates the two vertical rods at the same time; one rod extends up into the door head, and the other extends down into the door sill. It is similar in appearance to an "espagnolette

Espagnolette

An espagnolette is a locking device, normally mounted on the vertical frame of a French door or casement window. A handle or knob is connected to a metal rod mounted to the surface of the frame, at about three or four feet above the floor. Operating the handle rotates the rod, which has hooks at...

" lock, but uses a half-round rod.

Shown from the backside, the parts of the crémone bolt gearbox (1) includes the lever or knob (3), which rotates the pinion gear

Rack and pinion

A rack and pinion is a type of linear actuator that comprises a pair of gears which convert rotational motion into linear motion. A circular gear called "the pinion" engages teeth on a linear "gear" bar called "the rack"; rotational motion applied to the pinion causes the rack to move, thereby...

 (4), engaging the pair of rack gears

Rack and pinion

A rack and pinion is a type of linear actuator that comprises a pair of gears which convert rotational motion into linear motion. A circular gear called "the pinion" engages teeth on a linear "gear" bar called "the rack"; rotational motion applied to the pinion causes the rack to move, thereby...

(2), which then push the two half-round bars (5) upward and downward at the same time (only the bottom bar is shown), into the sockets.