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).

The representation of geminate consonants remains a controversial topic in phonological theory. In a skeletal theoretic approach, for example, a geminate is represented as bipositional: a single root node multiply linked to two skeletal positions, as in (1) (see e.g. Clements & Keyser 1983, Levin 1985).

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Conversely, in moraic theory, geminates are inherently moraic (see e.g. Hayes 1989, 1995, Davis 1994, 1996). Thus, an underlying geminate consonant differs from a single consonant of the same quality in terms of a mora, as shown in (2):

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It is noteworthy that evidence bearing on the representation of geminates has thus far come predominantly from the patterning of intervocalic geminates, where the first part of a geminate occurs in coda position. Discussion of syllable-initial geminates, on the other hand, has received little attention in the literature.

Syllable structure in Piro, an Arawakan language spoken in Eastern Peru, exhibits properties that are of considerable interest. First, Piro allows three-consonant clusters in the syllabic onset (Matteson 1965) but a vowel-deletion rule is blocked to avoid such clusters, which makes it difficult to simply attribute the blocking to well-formedness constraints on syllable structure (cf. Kisseberth 1970a, b). Second, the onset consonant clusters are not governed by the Sonority Sequencing Principle (Selkirk 1984a, Clements 1990). The fact that all consonants except the prevocalic ones are described by Matteson (1965) as syllabic seems to obviate the need to posit complex onsets. However, these syllabic consonants do not participate in any phonological rules, including word stress and phrase-level rhythmic rules. If they are considered extrasyllabic throughout the phonology, they become exceptions to our general conceptions of extra-prosodicity, stray erasure and prosodic licensing (Itô 1986, 1989), since numerous such extrasyllabic consonants are not at the peripheral position of a well-defined domain but persist to be present throughout the derivation. Third, loss of an onset or unsyllabified consonant can lead to compensatory lengthening (henceforth CL), contradicting the moraic conservation account of CL (Hock 1986, Hayes 1989). Fourth, as Piro has no underlying long segments nor evidence for a heavy closed syllable, it becomes a counterexample to the typological prediction that CL occurs only in languages that have a pre-existing vowel-length contrast (de Chene & Anderson 1979) or syllable-weight contrast (Hayes 1989).

Harmony is a phonological pattern in which a sequence of vowels and/or consonants must have the same specification for a particular feature, descriptively referred to as the harmonic feature. In the clear cases, whenever the harmonic feature occurs in a word, the expected manifestation of the harmony pattern is also encountered. For example, in Capanahua (Loos 1969) where [nasal] is the harmonic feature, a nasal consonant always shares nasality with a preceding sequence containing vowels, semivowels and/or laryngeals.