Getting the measurements and the proportions right

Wider than feels necessary. A narrow mat reads as a mistake, and a generous one reads as a decision. As a starting point, make the border wider than the frame is, and weight the bottom edge slightly. The calculator will do this for you.

Because the eye reads the centre of a frame as slightly above the true centre. Cut all four borders equally and the picture looks like it is sliding downwards. Adding roughly 20 percent to the bottom corrects it.

Almost never, and it is not worth trying. Ready-made frames are cut to photographic paper sizes and an original is whatever size the artist made it. Forcing the fit means either trimming the work or leaving it loose in the rebate.

Usually, but for structural reasons as much as visual ones. A large frame has to be stiff enough not to bow across the span, and that means a deeper section, which reads wider from the front.

Measure the sight size, the part that will actually show once framed, not the outer edge of the stretcher or the sheet. Measure both diagonals too, since a stretched canvas is rarely perfectly square, and note which measurement is larger before you order. See how to measure a painting for a frame.

Usually a gap close to the width of the moulding itself, so the panels read as one composition rather than as separate pictures with a wide margin between them. A gap much wider than that starts to break the set apart visually. See framing a diptych or triptych.

Not really. Ready-made frames follow photographic paper sizes, and an original painting is whatever size the artist stretched or cut it to, so the two rarely line up. Most originals need a frame cut to the exact piece rather than bought off a shelf. See standard vs custom frame sizes.

Yes. Enter the artwork's height and width and it returns every cut dimension, mat border, and outer frame size, with a scaled preview so you can see the proportions before you order anything or cut a single piece of board. See frame and mat size calculator.