This is the wood or metal moulding that surrounds the art package. This is the most important element, and the only one always present. An oil painting on canvas, for instance, will probably have only a frame. A photograph, on the other hand, will usually use several of the elements- frame, glazing, mat, and backing.

Choose a profile (shape) that you find pleasing, and has enough rabbet or channel space to contain all the elements you plan on using. If for instance you are using:
Acrylic glazing (1/8th inch thick) and
4 ply window mat (1/16th inch), and
Foam core backing (3/16th inch)
You’ll need at least 3/8th inches of rabbet space:
(1/8th + 1/16th + 3/16th = 3/8th inch).

Frames are ordered in inches and fractions. The size of frame you order depends on your art package. Are you framing a canvas and nothing else? You will measure the canvas from top to bottom and side to side. Are you framing a photo in a window mat? Measure or calculate the mat dimensions from top to bottom and side-to-side. The frame will be chopped to fit those dimensions- the rabbet or channel space will measure slightly larger than your art package.