There are three main types of replacement windows:

Flush Fin (also called Z-bar)
Typically used in California and other markets where aluminum windows are in place and the exterior is stucco or masonry. These windows have a large exterior flange that is designed to conceal the existing window’s frame.

Block Frame
Mainly found in markets where wood windows are in place and the exterior is siding or brick. The original wood perimeter frame is left in place and must be in good condition.

New Construction Nail Fin
Most standard-size new construction windows have a nail fin around the exterior frame. A nail fin is a thin piece of metal extending from the window itself that goes all the way around the perimeter of the window frame.