Incomplete neutralisation presents a problem for classical modular feed-forward grammars: it results in surface phonetic distinctions between phonologically neutralised segments. This paper argues for a model of incomplete neutralisation using two independently motivated theoretical devices: paradigm uniformity and weighted phonetic constraints. A case study is presented, showing that Japanese monomoraic lengthening results in incomplete neutralisation: when monomoraic nouns with short vowels are lengthened to fill a bimoraic minimality requirement, they reach a duration intermediate between that of unlengthened short vowels and underlyingly long vowels. The Japanese case has properties distinct from other classically cited examples of incomplete neutralisation such as final devoicing, which are not predicted by previous theories of neutralisation. The Weighted Paradigm Uniformity theory of incomplete neutralisation is shown to make four unique predictions, and is argued to better capture the typology of incomplete neutralisation.