Casing

Also called trim, architrave

The molding that frames the window on the interior wall.

Casing is the trim that surrounds a window on the finished wall. It covers the gap between the window frame and the wall surface. The piece across the top is the head casing; the two down the sides are side casings.

Casing is decorative as much as functional. Its profile, width, and corner joints do a lot to set the style of a room.

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Awning window, from inside

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Casement window, from inside

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Double-hung window, from inside

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Two double-hung windows in one opening

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Double-hung window, in section

Related

JambThe vertical side members of the window frame.
HeadThe horizontal top member of the window frame.
StoolThe interior ledge at the bottom of the window.
ApronThe trim board on the wall directly beneath the stool.
BrickmouldThe molded trim that frames the window on the outside wall.