Why a high-speed trace doesn't use its whole conductor cross-section — and why "it's just skin effect" is only half the story once a second conductor is nearby.
Every AC-current-crowding conversation eventually gets flattened into one phrase — "skin effect" — as if it explains everything happening in a high-speed trace, a transformer winding, or a multi-conductor cable. It doesn't. Skin effect is real and it happens to everyconductor carrying high-frequency current, all on its own. But the moment a second conductor is placed nearby — which describes almost every real signal path, since return conductors and neighboring traces are everywhere — a second, additional effect kicks in: proximity effect. It's not a rename of skin effect. It's an extra layer of current redistribution stacked on top of it, caused specifically by the neighboring conductor's own field, and it responds to design choices (spacing, layout) that skin effect never will.