Developmental patterns and interrelationships of various informal skills, reading and writing of numerals, and a range of base ten/place value concepts/skills were examined in a study involving 78 primary school children from four schools who were individually examined in a standardized interview. A total of 23 tasks were administered and scalogram, individual protocol, and error analyses were performed. Results indicate that the children learned to read and write numerals in a step-like fashion. For example, even though children wrote smaller terms correctly, they wrote larger, unfamiliar terms as they sound (e.g., 20090 for "two-hundred ninety"). Zeros caused many errors. While reading numerals preceded writing numerals for terms to 20, there was no consistent developmental relationship between these skills for larger terms. A "next-by-ten" elaboration of the mental number line appeared to underlie decimal ability. Base ten equivalents and place value appeared to be basic decimal knowledge, while operating with multiples of ten and an appreciation of the structure of the number system appeared to represent a deeper knowledge. Most second graders and even many third graders had only an imprecise appreciation of the repetitive pattern of the number system at the three digit level. (Author/RH)