The obligatory contour principle (frequently abbreviated OCP) is a hypothesis in autosegmental phonology that states that (certain) consecutive identical features are banned in underlying representations.
The principle is part of the larger notion of horror aequi, that language users generally avoid repetition of identical linguistic structures.
The locus classicus of the OCP is Leben (1973), in which it was formulated as a morpheme-structure constraint precluding sequences of identical tones from underlying representations.
Odden (1986) showed that, contrary to the contemporaneous assumption that constraints were inviolable, an examination of African tonal systems reveals many apparent surface violations of the OCP.
Yet many issues as to its precise formal character remain: locality - what is the domain of the OCP (i.e. strict adjacency, etc.)