Lesson objective: Understand the commutative property of addition with three addends.
Students bring prior knowledge of the commutative property of addition from 1.OA.B.3. This prior knowledge is extended to three addends. A conceptual challenge students may encounter is thinking they need to add the numbers in the order they appear or always start with the larger number when there may be a more efficient strategy.
The concept is developed through work with a number line, which helps support this concept by showing the different parts (addends) and the whole (sum) in various orders.
This work helps students deepen their understanding of equivalence, because changing the order of any number of addends does not change total.
Students engage in Mathematical Practice 7 (Look for a make use of structure) as they recognize that the order of the addends doesn't matter. Any way they order them, they will come up with the same sum.
Key vocabulary:

addend: one of the numbers in an indicated sum of two or more numbers (e.g., 4 + 3 + 1 = 8; 4, 3, and 1 are addends)

addition: a mathematical operation of combining two or more numbers into a total or sum

number line: a line on which a point represents a value or number

order: to place numbers or objects in a sequential arrangement (e.g., least to greatest or heaviest to lightest)

sum: the result when two or more quantities are added

total: the whole amount or value
Special materials needed:
 number lines (020)
 schedule template (optional)