Lesson objective: Extend understanding that length can be measured with non-standard length units when length units are the same-size, when they have no gaps between them, when they do not overlap, and when they are aligned.

Students bring prior knowledge of measuring length from K.MD.A.1 and 1.MD.A.1. This prior knowledge is extended to measuring accurately with length units as students identify the need for a measuring rule about alignment of length units. A conceptual challenge students may encounter is understanding the term "alignment."

The concept is developed through examining worked examples, which provides students with the opportunity to analyze the need for a rule about alignment when measuring with length units.

This work helps students deepen their understanding of equivalence because the number of same-sized length units that are aligned with no gaps or overlaps is equal to the length measurement of the object.

Students engage in Mathematical Practice #2 (reason abstractly and quantitatively) as they measure with non-standard length units and make approximations when the object being measured and the length units do not exactly align; Mathematical Practice #5 (use appropriate tools strategically) as they use non-standard length units to measure length; and Mathematical Practice #6 (attend to precision) as they strategically manipulate non-standard length units to find the length measurement of an object. Students examine worked examples and determine the reason for the inaccurate length measurements due to alignment issues.

**Key vocabulary:**

- accuracy
- alignment
- gaps
- length measurement
- length units
- overlaps
- same-size
- span

**Special materials needed:**

- non-standard length units (blocks, building cubes, paper clips)
- items to measure (e.g. strips of paper)