Children who use AAC often experience difficul- ties expressing multi-word messages (e.g., Smith & Grove, 2003). When relying on aided AAC sys- tems (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). A number of factors may contribute to the pre-dominant using of single-symbol messages by children using aided AAC. First, a lack of communicative efficiency may be partly to blame. One way of speeding up the relatively slow process of communicating via AAC is to use shorter messages (Smith & Grove, 2003; Sutton, Soto, & Blockberger, 2002), and another way to improve efficiency is to co-construct messages; that is,