Even accounting for the difference between western and eastern windows manufacturers, many windows have odd sizes. Why is that?

The “rough opening” is the framed opening in your wall for the window to fit in. Rough openings are usually sized 1/2” larger than the height and width of the window’s frame size to allow room to shim the window in case the opening is not perfectly square. However, some manufacturers recommend a rough opening size different from this rule of thumb — consult with any member of our sales staff to verify the proper rough opening for the window you are planning on using.