Much school mathematics is devoted to teaching concepts and procedures based on those units that form the core of whole number arithmetic (ones, tens, hundreds, etc.). But other topics such as fractions and decimals demand a new and extended understanding of units and their relationships. Behr, Wachsmuth, Post, and Lesh (1984) and Streefland (1984) have noted how children's whole number ideas interfere with their efforts to learn fractions. Hunting (1986) suggested that a reason children seem to have difficulty learning stable and appropriate meanings for fractions is that instruction on fractions, if delayed too long, allows whole number knowledge to become the predominant scheme to which fraction language and symbolism is then related.