Young Blonde Girl In Front Of The Window With Flowers

The lites in a window sash are divided horizontally and vertically by narrow strips of wood or metal called muntins. More substantial load bearing or structural vertical dividers are called mullions, with the corresponding horizontal dividers referred to as transoms.
In the United States, the term replacement window means a framed window designed to slip inside the original window frame from the inside after the old sashes are removed. In Europe, however, it usually means a complete window including a replacement outer frame.
The U.S., term new construction window means a window with a nailing fin designed to be inserted into a rough opening from the outside before applying siding and inside trim. A nailing fin is a projection on the outer frame of the window in the same plane as the glazing, which overlaps the prepared opening, and can thus be 'nailed' into place.
In the UK and mainland Europe, windows in new-build houses are usually fixed with long screws into expanding plastic plugs in the brickwork. A gap of up to 13mm is left around all four sides, and filled with expanding polyurethane foam. This makes the window fixing weatherproof but allows for expansion due to heat.