Given two linguistic categories C1 and C2, each with its values; then the marked value of C1 will show at most as much differentiation in terms of (the values of) C2 as its unmarked value.

An example from morphology: the Latin indicative distinguishes six tenses, the subjunctive five (it lacks a future).

An example from phonology: French has 11 or 12 oral vowels, but only 3 or 4 nasal vowels.

Brøndal, Viggo 1940, "Compensation et variation, deux principes de linguistique générale." Scientia 67:101-109.