diphthong or long vowel and syllable structure can be analysed as CCV when or is analysed as a C. Orthography As stated above, vowel length is contrastive. What would be written phonetically as a: is represented by aa. All long vowels are written this way. All alveolars () are dental-alveolars. They are represented in the orthography by t, d, n and nd. The sound is represented by y. The complete orthography of Mbula is as follows: Syntax and word classes In an ideal grammar each classificatory word type would belong only to one category and in Mbula that is mostly the case. However, in the following three areas, word forms exist which are hard to nail down as one or the other: verbs and prepositions verbs and adverbs verbs and instrumental nouns The occurrence of a form in a wide range of conversational environments can result in the gradual loss of morphological features which are not appropriate to some particular conversational environments as well as the morphological gain of features which are appropriate to other conversational environments. This can mean ultimately either category shift of a word class or even just the general greying of the word classes as iron-clad categories. Verbs and prepositions Prototypical verbs and prototypical prepositions exist along a cline with verbs at the start, prepositions at the end, and multicategoried word types in the middle: forms inflected with the subject prefixes which function syntactically only as predicates in sentences forms not inflected with the subject prefixes which syntactically function only as predicates in sentences (the uninflected verbs discussed below in verbs) forms potentially exhibiting subject agreement inflection which function syntactically as both predicates in sentences and in serial constructions (the prepositional verbs discussed below) forms never exhibiting inflection and which function syntactically only as prepositions Verbs and adverbs Prototypical verbs and adverbs exist along a cline with verbs at the start, adverbs at the end and multicategoried word types in the middle: inflected verbs which never occur as modifiers within the predicate phrase and never occur in cosubordinate adverbial predications inflected verbs which never occur as modifiers within the predicate phrase, and can occur in either a preceding or following cosubordinate adverbial predication uninflected verbs which never occur as modifiers within the predicate phrase, but which can occur in either a preceding or following cosubordinate adverbial predication forms which can occur as modifiers in the predicate