A window mount is a matboard with an opening cut into it, allowing the artwork beneath to be seen. The opening is commonly called the window.
The window determines how the artwork sits within the surrounding border. Its size and position can influence the balance of the finished frame, particularly when an image has a signature, printed margin or important detail near its edges.
A carefully planned window can help draw attention to the artwork without distracting from it. The surrounding matboard also creates a visual transition between the image and the frame.
Window mounts are often used for photographs, prints, drawings and other works on paper. The opening and mounting method should be planned to suit the particular artwork, rather than simply following its outer dimensions.