Length (cm, mm, m)

Everything rests on two relationships: one tenth of 1 cm is 1 mm, and 100 cm is 1 m. Starting from Measuring with a Ruler, children restate lengths between cm and mm and between m and cm, then add and subtract within one unit and across units, including the cases that carry and borrow.

Writing 8 cm 5 mm as 805 mm rather than 85 mm stands out, since the cm part is not multiplied by 10 during the conversion. In subtraction that needs borrowing, 1 cm is never exchanged for 10 mm, so the mm parts are subtracted smaller from larger and the mm in the answer comes out too big.

From a stage that only compared the lengths of objects, this unit moves on to expressing length as a number. The sense of lining up ends to compare, built in Comparing Length, becomes the foundation for measuring with units such as cm and mm, and the units keep growing into Length (km and Tape Measure) in grade 3, where km enters the picture.