The concepts "more" and "less" were analyzed into two meaning dimensions, "existence" (derived from children's early language) and "quantity," which were hypothesized to be developmentally related to acts of addition and subtraction. Two experiments tested two- and three-year-olds' comprehension of these concepts when initially equal or unequal rows were added to, subtracted from or left static. Addition and subtraction had little effect on Ss' comprehension of either term. Ss understood "more" first when number characteristics of the array were relatively large, suggesting "many" as an intermediate stage of meaning for "more." No evidence was found for "less" meaning "more," as others have claimed. Ss understood "less" later than "more," a difference attributed to the restricted use of "less" as "smaller in amount." (Author)