Default features and feature values are the most frequent for a language. In generative phonology, phonological rules were thought to operate on fully specified representations Nonlinear theories disagree, posing that only some features are specified. Underspecified features are ultimately filled in via development Contrastive Specification assumes features specs are stored in underlying reps when needed to contrast segments Radical Underspecification assumes that underlying reps include only predictable values for features characterizing the segments in an inventory Combinatorial Specification is discussed later, in section on Grounded Phonology. Defaults Substitute for Nondefaults Defaults often possible when nondefaults aren't When a nondefault isn't possible, the default feature, value replaces it. The unlinked feature or node can link up elsewhere.
Coalescence is an example.
Child had a constraint that onsets could contain only one consonant. I like this because it accounts for some of the input and processing factors that might be missing from, or at least not emphasized in, the other theories. Different languages can have different underspecified values for features See variability across children, too.