Applied Psycholinguistics 21 (2000), 473–486 Printed in the United States of America Relative clause sentence production using augmentative and alternative communication systems ANN SUTTON Universite ´ de Montre ´al TANYA GALLAGHER University of Illinois JILL MORFORD University of New Mexico NAVID SHAHNAZ McGill University ADDRESS FOR CORRESPONDENCE Ann Sutton, E ´ cole d’orthophonie et d’audiologie, Universite ´ de Montre ´al, C.P. 6128, Succursale Centre-ville, Montreal, Quebec H3C 3J7, Canada. Email: ann.sutton@umontreal.ca ABSTRACT Complex syntactic structures may be difficult to recognize when produced using augmentative and alternative communication (AAC) systems that do not contain grammatical markers. The present study investigated adult English speakers’ production of Subject and Object relative clause sentences using a picture/symbol-based AAC system with speech output. Most participants avoided the poten- tial ambiguity that resulted from the absence of grammatical markers. They followed spoken English word order when encoding Object relative clause sentences, but altered this order for Subject relative clause sentences. Most participants used constituent proximity to maintain the distinction between Subject and Object relative clause sentences. Results indicate the combined effects of underlying syntactic knowledge and pragmatic variables on the AAC constituent order patterns observed. Communication using augmentative and alternative communication (AAC) sys- tems such as picture/symbol displays and voice output devices differs signifi- cantly from spoken communication. This is recognized widely in the literature (Beukelman & Mirenda, 1998; Gerber & Kraat, 1992; Glennen & DeCoste, 1997; Kraat, 1985; Nelson, 1992), but our understanding of the underlying na- ture of these differences is less certain. Intrinsic variables, visual modality fac- tors, and AAC system constraints all may influence utterance forms produced using AAC systems. Intrinsic variables include differences in underlying knowl-  2000 Cambridge University Press 0142-7164/00 $9.50