Windows must be properly aligned along many different planes when installed into a rough opening. The window must be square, level and plumb to ensure a uniform finish, prevent water infiltration, and allow proper operation of moving window components and prevent premature failure.
Square: The window must sit freely and squarely in the window opening. Any pressure at any point on the frame can push the window out of square and compromise the seals, put stress on the glazing to the point of fracture and impede movable components.
Level: When the window is level along the horizontal axis no water pockets will be created in the frame of operating windows and the visual lines of the window will be in sync with the other lines of the house. When it is level along the vertical axis, all the seams around the interior perimeter of the frame will be uniform.
Plumb: When the window is plumb, or level from top to bottom, the integrity of the sill slope is maintained so it can shed water as designed. The weatherstripping will also engage properly and the window will operate correctly.