Which power input takes precedence?

Uno specs in the documentation tab here say:

The Arduino Uno board can be powered via the USB connection or with an external power supply. The power source is selected automatically.

As far as I can see though, it doesn't say which of those supplies is active if both are present.

What's the rule?

V-in takes over when the voltage is higher than USB supply plus the dropout voltage of the onboard 5volt regulator.
That dropout voltage could be ~1volt, depending on current draw. So about 6volt.
USB is switched off (with a mosfet/opamp) at >=6.6volt on V-in.
Schematics are on the Uno page on this site.

Thanks for the answer.

Schematics are on the Uno page on this site.

Yes I know: they're actually linked from the very page I linked to in my post. But if I could read a schematic to that level of understanding I wouldn't have needed to ask the question.