Dual-pane patterns, dimensions and hinge callbacks

The sibling skills explain the platform: reserved regions and arrangements in ../iphone-duo-adaptive-layout/SKILL.md, the hinge and scenes in ../iphone-duo-hinge-and-scenes/SKILL.md, design principles in ../iphone-duo-design-review/SKILL.md. This one is the applied layer: the numbers you'll want when mocking up or sizing assets, the exact shapes of the hinge callbacks, and the proven patterns for what to put in each pane — drawn from Apple's material plus a decade of dual-screen guidance from Microsoft (Surface Duo), Google (Android foldables) and Samsung (Flex Mode), which all converge on the same answers.

The numbers

From Apple's published specs. Use them for mockups, asset budgets and sanity checks — never as layout constants. Layout reads size class, safe area and reserved regions at runtime.