If a mounted picture where all the borders are equal is hung on the wall, an optical illusion called foreshortening happens, and the bottom border will appear to be narrower than the top and side borders. This is why when mounting a picture, it is good practice to add 0.5cm extra to the bottom border which counters this optical illusion, so the picture will look balanced and ‘correct’ when hung.
Measuring a Mount to Fit a Frame
When measuring a mount to fit a frame, you must always measure from the back, measuring the backboard or glass size, not the visible glass size at the front, which is smaller due to the rebate of the frame.
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