The painting looked balanced on the desk. Behind a window mount, the tree suddenly feels too close to the edge. The artwork has not changed, but the visible area around it has.
Before cutting the paper, use loose strips of plain card to test different window sizes. Keep the strips clean and avoid dragging them across a delicate surface. This temporary arrangement lets you change the border without making an irreversible decision.
Decide what belongs in view
Some paintings benefit from showing a clean paper margin. Others are composed to run to the edge of the visible image. Look for details that should not disappear beneath the mount: a signature, a fine branch or a deliberate wash boundary.
Try one arrangement, step back and photograph it for private comparison if useful. Then change only the opening. Comparing a new opening, different colour and wider frame at once makes it harder to understand what improved the balance.
Measure the opening and the outside separately
The mount’s outer size must suit the frame, while its opening determines how much artwork is visible. Those are two different measurements. Write both down, along with the paper size, before ordering anything described only by a standard frame dimension.
Ask the framer how the work will be supported and kept clear of the glazing. For valuable, fragile or sentimental artwork, discuss suitable conservation materials and reversible attachment rather than experimenting with ordinary household tape or glue.
Do not trim simply to make the painting fit a frame you already own. A different mount or frame may preserve choices that cutting would remove. If the paper has an attractive deckled edge, consider whether showing it is part of the presentation and ask how that affects mounting.
Leave the mock-up overnight if the choice feels uncertain. View it in the intended room at a normal distance. The useful border is the one that lets the painting read comfortably in its setting, not necessarily the narrowest border that will fit inside the nearest available frame.