You round a card at 16px. You round the thumbnail inside it at 16px too, because that's the design token. Then you squint at it and something is off — the gap between the two curves looks fatter at the diagonal than it does along the straight edges, and the inner corner reads as slightly pinched.

Your eyes are right. Two rounded rectangles only look parallel when their radii differ by exactly the distance between them.

The rule

inner radius = outer radius − gap

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