This unit was designed for grade one, but if it was not used in grade one consider inserting it into the grade 2 sequence because its purpose is to develop the open number line model—a representational model you will need as you work on addition and subtraction within 1000. The unit also introduces measurement with non-standard units to emerge the open number line model and prepares the way for examining 2-digit numbers, magnitude, number relations, and landmark numbers. This unit should be done BEFORE concerted work on addition and subtraction with double digits starts.