In nasal harmony systems, any major consonant class (excluding laryngeal glides) may fail to undergo nasalisation. This paper proposes to derive the neutrality of stops, fricatives, liquids and glides from the satisfaction of a special set of faithfulness constraints; each member of the set commands the presence of a particular segment class in a consonant inventory. The analysis requires that the constraint commanding the presence of stops be active in every grammar and invariably take precedence over the demands of nasal harmony. The explanation of stop neutrality forces a rethinking of the nature of constraints and how they interact. The paper argues that the constraints regulating the cross-linguistic manifestation of segment neutrality are either universal grammatical features (i.e. principles) or language-specific choices (i.e. parameters of variation). It also postulates that constraint interaction is regulated by a set of metaconditions that determine ordering; constraint ranking is not random.