A mat window is the opening cut in a mat to display the picture. We will assume that we are working with a standard size frame, which will normally be the case.

Cut the mat to the size of the frame. Frames are actually made an eighth of an inch larger to accommodate an exact size and allow for some error.

Measure the picture to determine the size of the window. Allow for at least 1/8 to ¼ inch overlap on each edge. Of course, anything larger is fine as well. Suppose we want to frame an 8x10 picture with a ¼ inch over lap. The window size is then 7 ½ x 9 ½.

Compute the distance from each edge of the mat to make the window cuts. This is done as follows. Write down the outside mat dimensions, e.g. 11x14 and subtract the corresponding window dimensions.

Now, we need to divide the left-over numbers in half since we want the same boarder to be symmetrical around the picture. To do this, convert each number to a simple fraction as follows:

Now divide by two. This is the same as multiplying by ½, which gives:

These numbers can now be converted back to a number and a fraction for making measurements and cuts. We have

That is, the mat boarders on the narrow dimension of the frame will be 1 ¾ inches and the mat boarders on the wide dimension of the frame will be 2 ¼ inches.

Turn the mat over with the good side down and outline the window. To do this, measure in from each side by the amounts computed above. Measure from an outside edge and mark on both the top and the bottom the appropriate distance and draw a line connecting the marks. When you are done marking, you will be ready to cut the window opening.