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The mat window is cut slightly smaller than the artwork so the edges are held rather than floating, then the frame opening is the window plus the border on all four sides. A 5-8 cm border suits most artwork, and many framers make the bottom border slightly deeper for visual balance.
Frame sizing
mat window = artwork - 2 x overlap; frame opening = window + 2 x border
So the mat holds the edges down and hides any irregular trim. 3-5 mm on each side is standard.
Optical weighting: an equal border looks bottom-light when hung at eye level, so adding 10-20% to the bottom looks more balanced.