SEQUENTIAL/PHONOLOGICAL CONSTRAINTS
All native speakers of a language know, unconsciously of course, that the phonemes of their language cannot be strung together in any random order to form words. In fact, the phonological system of their language determines which phonemes can begin a word, end a word, and follow each other. Very often, sequential constraints are biologically-driven. That is, there is a biological explanation for them. Since breathing is the primary function of human vocal organ, speech is considered to be its subsidiary function. Therefore, any form of speech that can interfere with breathing is impossible; hence, sequential constraints.
Native speakers of English, for example, tacitly know that English words can begin with such sequences as sp, sk, dr, tr, and str but not with such sequences as bp, pb, lbk, etc. This unconscious knowledge tells them that no English word will ever begin with a sequence that is phonologically constrained in their language. New words enter the language but they all conform to the sequential constraints of the language. Wog is phonologically possible and may be added to the English language in future as a new word but *wgo is phonologically impossible and will never enter the English language as a new word. In fact, it can safely be argued that wog is a potential word (a word to which no meaning has been assigned yet). Such potential words are sometimes called accidental gaps.
