An astragal is a moulding profile composed of a half-round surface surrounded by two flat planes (fillets

Annulet (architecture)

Annulets, in architecture, are small square components in the Doric capital, under the quarter-round. They are also called fillets or listels....

). An astragal is sometimes referred to as a miniature torus

Torus

In geometry, a torus is a surface of revolution generated by revolving a circle in three dimensional space about an axis coplanar with the circle...

. It can be an architectural

Architecture

Architecture is both the process and product of planning, designing and construction. Architectural works, in the material form of buildings, are often perceived as cultural and political symbols and as works of art...

 element used at the top or base of a column

Column

A column or pillar in architecture and structural engineering is a vertical structural element that transmits, through compression, the weight of the structure above to other structural elements below. For the purpose of wind or earthquake engineering, columns may be designed to resist lateral forces...

, but is also employed as a framing device on furniture

Furniture

Furniture is the mass noun for the movable objects intended to support various human activities such as seating and sleeping in beds, to hold objects at a convenient height for work using horizontal surfaces above the ground, or to store things...

 and woodwork.

The word "astragal" comes from the Greek and Latin for "vertebra," and the original astragals used in architecture were made in a beaded design, thereby resembling a vertebra.

An astragal is commonly used to seal between a pair of doors. The astragal closes the clearance gap. The vertical member (molding) attaches to a stile on one of a pair of doors (either sliding or swinging) against which the other door strikes, or closes to.

Exterior astragals are kerf

Kerf

Kerf and similar can mean:* kerf, the width of a cut* Kerf, a poetry collection by Peter Sanger* Kerala E.N.T. Research Foundation , a hospital at Kollam in Kerala in India...

ed for weatherstripping. Also flush head and foot bolt hardware is commonly mortised into the astragal to hold the inactive door in place, when both doors operate, at the top and bottom. It is related to Moulding

Molding (decorative)

Molding or moulding is a strip of material with various profiles used to cover transitions between surfaces or for decoration. It is traditionally made from solid milled wood or plaster but may be made from plastic or reformed wood...

.

Also known as “meeting stile seals,” the term can refer to the raised half-round overlap where pairs of doors meet, such as is the case with French doors. An astragal is designed to be applied to one or both doors of a pair at their meeting edges (meeting stiles). The astragal closes the clearance gap for the purpose of either providing a weather seal, ensuring privacy, preventing sound from leaking in or out of a room, minimizing the passage of light between the doors, or retarding the passage of smoke or flame during a fire.

Doors are typically the weakest link in any partition that is designed to block sound. This is often due to poor sealing around the perimeter of the door. Astragals, perimeter gasketing, drop seals and door sweeps can all be used to prevent sound from leaking through cracks around the door perimeter.