The principle of the permanence of the rules of calculation is contrasted with the concretization permanence principle. Both apply to situations where some arithmetical operation known to children for numbers of a certain kind is to be extended to include further numbers. (MNS)

The importance of teaching computational estimation is discussed, with note of its increased significance because of widespread use of calculators and the need to develop number sense. The reason for its failure to be taught is traced to the multiplicity of methods, and some detailed suggestions are therefore presented. (MNS)