Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. The Obligatory Contour Principle (OCP) is a phonological hypothesis that states that (certain) consecutive identical features are banned in underlying representations. 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, 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 (in this hypothetical language), the high tone of the one vowel spreads onto the other. Alternatively, one (or both) 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 got deleted at some point. Regardless, the OCP claims that there can not have been two consecutive high tones (nor two consecutive low tones, etc.) in the underlying representation of the morpheme, i.e. in the morpheme''s lexical entry.