As Peters (2001) has suggested, the young child's use of fillers seems to indicate awareness of distributionally-defined slots in which some as yet unidentified material belongs. One may view a filler as an emergent transitional form; as a slot that serves as an underspecified lexical entry for the accumulation of phonological and functional information; or as prosodic ‘sentence padding’. We trace the development of three fillers in one English-acquiring child through seven months, from their first appearance about 1;9 through their re-analysis as English functors about age 2;4. We show how the description of these fillers requires an elaboration of the current framework for describing the emergence of morphology, from a one-dimensional to a multi-dimensional model. Finally, we argue that the neglect of such transitional and under-defined elements gives a false picture of development, making it appear as if language development takes place in discontinuous steps.