WINDOW MAT A mat board with a hole cut out, through which the art is visible. Commonly used with photographs and prints. The standard mat thickness for a window mat is 4 ply, or about 1/16th inch, although 8 ply is sometimes used for very large pieces. Double matting is also common; this is usually two 4 ply mats with the top mat window cut at least 1/4 inch larger on all sides.

There are three sets of dimensions that apply to a window mat:
– THE OUTSIDE DIMENSIONS, which will correspond with the size of your frame, glazing and backing.
– THE WINDOW DIMENSIONS, which is the size of the hole. This is determined by the size of your artwork. The window can crop the piece or extend beyond the image area, but it always extends at least a 1/4 inches into the paper. You do not want the edge of the paper to be seen when using a window mat.
– THE BORDERS, which are the most subjective. A rule of thumb says borders should be at least 2 inches. Another rule of thumb says that you cannot have too much of a border. Side borders should always the same size. It is common to “weight” the bottom border, or make it larger than the top. This is because of an optical illusion that causes a bottom border the same size as the top border to appear smaller. This phenomenon is more distinct as pieces grow larger, and it is also more common to pictures oriented vertically. It is not, however, incorrect or problematic to simply center the image. Also, borders do not necessarily need to be consistent. A picture will look fine with borders that are, for instance, 3 1/2 inches top, 3 3/4 inches bottom, and 3 inches sides.

You can figure your borders two ways, but either way, you need to know the window size to start.

The first method starts with the frame size, for instance a 24” X 20”:

Standard Frame Size Formula
Frame Size	minus	Window	=	÷ 2	=	Borders
Height 24" Width 20"	minus	14" 11"	=	10" ÷ 2 9" ÷ 2	=	5" Top, 5" Bottom 4.5" Side, 4.5" Side
Height 24" Width 20"	minus	13" 9"	=	11" ÷ 2 11" ÷ 2	=	5.5" Top, 5.5" Bottom 5.5" Side, 5.5" Side
Height 24" Width 20"	minus	13.5" 8.5"	=	10.5" ÷ 2 11.5" ÷ 2	=	5.25" Top, 5.25" Bottom 5.75" Side, 5.75" Side

The second method starts with the borders you want:

Custom Frame Size Calculation Forumla
Window Dimensions	+	Border Dimensions	=	Frame Size
Height 15" Width 12"	+	3" Top + 4" Bottom 3" Side + 3" Side	=	22" Height 18" Width
Height 20" Width 16"	+	5" Top + 5" Bottom 5" Side + 5" Side	=	30" Height 26" Width

A series of different size pieces displayed together look best with a standard frame size and variable borders. Start with an optimal frame size and the dimensions of your windows, then calculate the borders with the “Standard Frame Size Formula” above. For a weighted bottom, subtract a little from the top and add it to the bottom.
A stand-alone or uniquely sized piece may benefit from a custom size frame. To determine that size start with the dimensions of the window, then choose your ideal border sizes. Use those dimensions and the “Custom Frame Size Calculation Formula” above to determine the size of your frame.

A GENERAL NOTE ON ALL MAT BOARD. There are many different varieties of mat board, usually made from either wood pulp (paper), or cotton fiber (rag). The important detail is the acid content of the mat, since acid can bleed from the mat and discolor artwork. Westfall Framing handles only 100% cotton rag mat, acid-free, solid core (same material throughout), manufactured to the traditional conservation standards.