Viewing the number system as a complex of interrelated words, three studies are described: (1) a study of the difficulty of simple addition and subtraction in different decimal positions; (2) a study of variations in the number of digits in the addend or minuend; (3) a study of the effect of irregular morphemes occurring in some number words. Findings proposed were (1) arithmetic tasks involve sequences of operations of reading and arithmetic called shifts; (2) different arithmetic performances require different shifts and thus some performances require more time to complete. (JG)