Evidence is presented from an experimental study with 21 children ages 4 to 5 years suggesting the coincident emergence of certain DPs as barriers to wh-movement and as separate binding domains. It is argued that the default assumption for children's grammar may be to assume NP is the maximal projection for a structure until a DP proves justified. When a DP emerges, it simultaneously becomes a barrier to movement and a separate domain for binding. This account makes contact with several aspects of current grammatical theory of the NP-DP distinction, and its implications for acquisition are explored.