The "frame width right" is measured on the outside of the window, from the inside of the open window frame to the fixed edge of the window cut-out (brickwork) on the building. Seen from the inside, this is therefore on the right-hand side.
The "frame width left" is measured on the outside of the window, from the inside of the open window frame to the fixed edge of the window cut-out (brickwork) on the building. Seen from the inside, this is therefore on the left side.
