Choose the borders you want to see.

Define the border from the window.

The visible mat border is measured from the opening edge to the mat’s outside edge. Paper coverage is hidden behind the opening and is a different measurement. On a centered layout, the two horizontal borders share the width left over after subtracting the window; the two vertical borders share the remaining height.

For an 11 × 14-inch board and a 7.75 × 9.75-inch window, each left/right border is (11 − 7.75) ÷ 2 = 1.625 inches. Each top/bottom border is (14 − 9.75) ÷ 2 = 2.125 inches. Equal opposite borders do not mean all four borders have the same width.

Understand bottom weighting.

A bottom-weighted layout gives more board below the window than above it. People may prefer its visual balance, but it is an aesthetic choice. The tool does not claim a scientifically correct ratio or a universally professional border width.

In our existing-board mode, ‘Extra bottom compared with top’ is the difference between the bottom and top borders. With a 0.5-inch difference in the example above, top = (14 − 9.75 − 0.5) ÷ 2 = 1.875 inches; bottom = 1.875 + 0.5 = 2.375 inches. The board height and window height stay the same.

Keep the sheet behind the board.

Increasing the bottom border in a fixed board moves the opening upward. This reduces the top border, which still needs to contain the paper extending above the window. If that border becomes smaller than the top coverage, the paper would extend beyond the board in this model, and the planner rejects the arrangement.

A larger bottom border does not itself expose a signature. Visibility is controlled by the window and its position relative to the paper. Use the visibility check for that question; use weighting for the visible border balance.

Build outward from chosen borders.

If you are not reusing a fixed board size, custom-borders mode calculates outside width as window width + left border + right border, and outside height as window height + top border + bottom border. You can specify all four borders separately.

An outside result is not a stock-frame recommendation. Confirm the practical minimum border, cut geometry, available board, material thickness and frame fit with your supplier. Very narrow borders may be mathematically possible without being suitable to manufacture or mount.

Compare a small set of meaningful options.

Start with centered borders, then compare one alternative while holding the opening constant. Check the composition in the intended orientation and write down the exact window position. Choose the result for the actual artwork and display, rather than relying on an unexplained rule of thumb.