If we compare the contrastive potential of different phonological contexts in any language, it usually does not take long to establish that the distributional spoils are unevenly divided. Each context typically displays its own subsystem of oppositions which may be bigger or smaller than those associated with other contexts (cf. Twaddell 1935). The traditional term NEUTRALISATION describes the relation between a defective subsystem and one that is distributionally better endowed.

The failure of a position to sustain a particular contrast can manifest itself in one of two ways, as Trubetzkoy was among the first to point out (1939: 209ff). Under ASSIMILATIVE NEUTRALISATION, the phonetic interpretation of the position with respect to the relevant contrast is determined by the melodic content of an adjacent position. This type of pattern is evident in vowel harmony, where the quality of a harmonising vowel is wholly or partially dependent on that of the dominant vowel within the domain. It is also to be seen in the assimilative suspension of consonantal contrasts. For example, in coda–onset interludes consisting of full or partial geminates, the phonetic interpretation of one position is wholly or partially dependent on that of the other.

REDUCTIVE NEUTRALISATION, on the other hand, refers to a situation in which restrictions on the melodic content of a position operate independently of contrasts in neighbouring positions. In vowel systems, for example, it is quite usual to find that the maximal inventory of oppositions is restricted to prosodically prominent nuclei, while shrunken subsystems of various shapes and sizes show up in weak positions. In its most extreme form, syncope, this results in a nuclear position being gutted of all melodic content. In the case of non-nuclear positions, contrastive potential can be curtailed by sonority sequencing constraints and by consonantal lenition processes which neutralise distinctions of manner (as in vocalisation and spirantisation) or place (as in debuccalisation).