This study reports on a strictly-cognitive and symptomatic approach to the treatment of phonological disorders, by an effect which can also be reproduced in most normally- developing children. To explain how this works, it is necessary to address certain asymmetries and singularities in the distribution of children's speech errors over the whole range of development. Particular words occasion particular errors. In early phonology there is 'fronting' with Coronal displacing Dorsal, and harmonies where Coronal is lost. In the middle of phonological acquisition, the harmonic pattern changes with coronal harmony coming to prevail over other forms. As well as these asymmetries, there is also the case of harmonic or migratory errors involving the property of affrication, but not the affricate as a whole, i.e. ignoring the property of voicing. Many of these asymmetries and singularities and the harmony or movement of affrication are described here for the first time. They are all difficult to explain in current theoretical models, especially in 'bottom-up' models. On the basis of the 'top-down' notion of 'parameters' from recent work in phonology, I shall assume that: A) finite learnability has to be ensured; B) there can be no privileged information about the learnability target; and C) phonological theory and the study of speech development (normal and otherwise) have an object in common. I shall propose: A) a Parameter Setting Function, as part of the human genome, possibly a defining part; B) Phonological Parapraxis', as a way of characterising the generalisations here about incompetent phonology by the general mechanisms of floating' and 'non-association'; C) a Stage (_n-1) as a necessary construct in the theory of acquisition, typically not reached before 8;6; D) a' Representability Inspection' relating normal competence to Chomsky's Articulatory/ Perceptual interface', sensitive to a relation between featural properties such as roundness or labiality and prosodic properties such as the foot and syllable; E) a syndrome. Specific Speech and Language Impairment, SSLI, extending the notion of Specific Language Impairment, SLI.I shall hypothesise that: A) segmental and suprasegmental representations interact; B) the phonological learnability space is uniform and consistent; C) it is the very minimality of the learnability system which makes it vulnerable to SSLI. This: A) side-steps the implausible inference that development proceeds by the loss of 'processes'; B) accounts for at least some of the asymmetries noted above; C) lets parameters set' a degree of abstract exponence; D) makes it possible to abolish 'processes' such as fronting, lisping, consonant harmony, in favour of successive degrees of imprecision in the parameterisation; E) provides a conceptual mechanism for the cognitive and symptomatic therapy, mentioned above: the therapy effects an increase in the set of phonological structures which are 'representable' by the child
