This article investigates prototypically attributive versus predicative adjectives in English in terms of the phonological properties that have been associated especially with nouns versus verbs in a substantial body of psycholinguistic research (e.g. Kelly 1992) - often ignored in theoretical linguistic work on word classes. Inspired by Berg's (2000, 2009) 'cross-level harmony constraint', the hypothesis I test is that prototypically attributive adjectives not only align more with nouns than with verbs syntactically, semantically and pragmatically, but also phonologically - and likewise for prototypically predicative adjectives and verbs. I analyse the phonological structure of frequent adjectives from the Corpus of Contemporary American English (COCA), and show that the data do indeed support the hypothesis. Berg's 'cross-level harmony constraint' may thus apply not only to the entire word classes noun, verb and adjective, but also to these two adjectival subclasses. I discuss several theoretical issues that emerge. The facts are most readily accommodated in a usage-based model, such as Radical Construction Grammar (Croft 2001), where these adjectives are seen as forming two distinct but overlapping classes. Drawing also on recent research by Boyd & Goldberg (2011) and Hao (2015), I explore the possible nature and emergence of these classes in some detail.