INTRODUCTIONChildren who use AAC often experience difficulties expressing multi-word messages (e.g., Smith & Grove, 2003). When relying on aided AAC systems (e.g., communication boards, voice output systems), many young children predominantly use single symbols (e.g., photos, line drawings) to communicate (e.g., Sutton & Morford, 1998). The critical shift from expressing single- to multi-word messages, which occurs in typically developing children at approximately 18 months of age (Owens, 2001), marks the emergence of syntax and the onset of generative language (Paul, 1997).