Abstract: A foldable nail fin for a window comprising an inner end which is normally positioned in a kerf formed in the window frame. The nail fin is normally positioned in a substantially flush condition with respect to the window frame. At the building site, the nail fin may be folded from its substantially flush position to an outwardly extending position so that the nail fin may be secured to a building surface extending around a window opening formed therein. The folding of the nail fin is enhanced through the use of a fold line formed in the nail fin which is located closely adjacent the inner end thereof.