The undersill trim is installed under every window above the ground line. It conceals the top edge of the vinyl siding and is also used at the soffit line.

Do you need Undersill trim?

It needs to be under and window sills to keep it from buckling. I also use it at the top of the wall inside the J. I use a snap lock tool to punch rectanguler shaped hole that leave a tab that locks into the undersill trim so it never comes loose.

Is Undersill and finish trim the same?

Utility Trim: a piece of trim used any time the top lock has been removed from the siding, to secure a siding panel. Also referred to as “undersill” or “finish” trim; double utility trim is also available.