This curriculum unit investigates how third-grade students develop a robust understanding of the base-ten number system through the use of landmark numbers. Authored by Susan Jo Russell and Andee Rubin, this resource is part of the Investigations in Number, Data, and Space series. It provides educators with a structured framework to help students move beyond rote memorization by utilizing visual models and logical problem-solving strategies. The material focuses on building numerical fluency within the context of the hundreds and thousands.

Educators frequently cite this unit as a foundational resource for implementing inquiry-based mathematics in the classroom. Experts highlight the curriculum's effectiveness in bridging the gap between abstract numerical concepts and concrete student comprehension.