Nailing fins are flanges on the outside of vinyl and aluminum windows that sit flush agains the sheathing when the window is in place. They provide a convenient way to attach the window, but wood windows, as well as some aluminum and vinyl ones, don't have these fins. When the fins are absent, you have to drive fasteners through the window frame and into the building framing. It isn't difficult to get good results, provided that you prepare the window rough opening, making sure it's the right size, and level the window before you attach it.

• How To Install Vinyl Baseboard

How To Install Vinyl Baseboard

Nailing fins are flanges on the outside of vinyl and aluminum windows that sit flush agains the sheathing when the window is in place. Job Done Right. He then nails the window flange onto the framing using the manufacturer’s recommended nail spacing. He finishes setting the nails by using a nail set (turned around for more surface area on the nail head) to avoid damaging the vinyl window frame with his hammer.