Nappe: the spoon test

The only consistency measurement in classical cooking that needs no instrument, and the one every sauce recipe assumes you know.

Dip a metal spoon into the sauce and lift it out. Sauce should cling in an even film rather than running off in streaks.

Draw a finger through the film. If the line holds open with clean edges, the sauce is at nappe. If it closes back over, keep reducing.

Judge it hot. Every sauce thickens as it cools, so a sauce that is perfect in the pan will be stiff on the plate. Stop one shade thinner than you want it.