A commonly held conception within phonology is that no morpheme is allowed to contain two consecutive high tones. If two consecutive high tones appear within a single morpheme, then some rule must have applied. Maybe one of the surface high-tone vowels was underlyingly high-toned, while the other was underlyingly toneless. Then, since all vowels must have tone at the surface, the high tone of the one vowel spreads onto the other. Alternatively, one of the vowels may have started out low-toned and become high-toned due to the application of some rule; or perhaps there was a low tone between the two high tones that gotdeleted at some point. Regardless, the OCP claims that there can not have been two consecutive high tones in the underlying representation of the morpheme, i.e. in the morpheme's lexical entry.
History
The locus classicus of the OCP is, in which it was formulated as a morpheme-structure constraint precluding sequences of identical tones from underlying representations. In autosegmental phonology, with articulated conceptions about associations between featural melodies and skeletal units, moraic phonology, the OCP was considered to be relevant to adjacent singly linked melodies but not to doubly linked melodies. The OCP in this 'rules and constraints' era was no longer simply a constraint on underlying forms, but also began to play a role in the course of a phonologicalderivation. proposed that the OCP can actively block the application of or repair the output of phonological rules, while in, Moira Yip attempted to extend the role of the OCP to trigger the application of rules as well. However, there was also a strong opposition to the OCP as a formal constraint in phonological theory, headed by David Odden. 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. A lively debate continued between John McCarthy and David Odden for several years, with each adding an extra 'anti-' to the title of the previous article of the other - e.g. "Anti anti-gemination and the OCP", a reply to.
Debate
In Optimality Theory , the OCP has been again redefined as a violable constraint. Yet many issues as to its precise formal character remain: locality - what is the domain of the OCP and how is the domain represented in the theory; near-identical sequences - many languages show an OCP-like resistance to sequences of segments that differ in just one distinctive feature; is this the effect of the OCP, some other constraint? If the latter, how is this constraint formally related to the OCP; status as an OT constraint - is the OCP a single constraint, or is it the local self-conjunction of markedness constraints ? These and other issues related to the OCP continue to be hotly debated in phonological theory.
A particular instance of the OCP is Meeussen's Rule, named after the Belgian Bantu specialist A. E. Meeussen, which has been used to explain how a sequence HH tones becomes HL in various Bantu languages.
