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.
Source: [Dev.to](https://dev.to/sgbp/the-concentric-border-radius-rule-why-nested-rounded-corners-look-slightly-wrong-3hog)