In this section, students measure the length of objects by iterating length units. They learn the conventions of length measurement and represent their measurements with a number and the name of the length unit. They understand that the length measurement of an object is the number of same-size length units that span it without gaps or overlaps.

Students use manipulatives, such as connecting cubes, paper clips, and centimeter cubes as length units. Other units of measurement that would yield a whole number of length units are also suggested.

It is important for students to measure lengths in whole units as they are developing the idea that the number of units for the same length would not change when measured by different people, as long as they are using the same units to measure.

Students expand their counting and number-writing skills to 120 as they use centimeter cubes to measure lengths that are longer than 99 length units. Along the way, students consider groups of 10 and see that 10 tens is 100. A hundred is not discussed as a unit in grade 1, but the written notation is introduced, so students can read and write the numbers 100–120.

In this section, students transition from direct comparison of lengths to indirect comparison. They reason about how they can compare the lengths of objects that cannot be easily lined up. 

Students begin this transition by ordering the length of three objects directly. Next, they compare two objects by using the length of a third object. 

For example, students may compare the pencil and the pen directly by lining them up and see that the pencil is shorter than the pen. Then, they may compare the pen and highlighter directly and see that the highlighter is longer than the pen.

Without comparing the pencil and highlighter directly, students could say that:

• The pencil is shorter than the highlighter because it is shorter than the pen.
• The highlighter is longer than the pencil because it is also longer than the pen.

At the end of the section, students practice using a third object as a tool to compare the lengths of objects that are very difficult to line up. For example, they use a string to compare the length of one side of a desk and the length of one of its legs.

The work here prepares students to iterate length units to measure objects in the next section.

In this section, students solve all types of story problems with unknowns in all positions. They interpret and write addition and subtraction equations that represent the problems.

Students begin by building and comparing concrete objects to solve Compare problems in the context of measurement. These problems involve Bigger or Smaller Unknown, a new problem type for students, and can be represented by diagrams such as shown here. 

Next, students solve Take From problems with unknowns in all positions with a focus on Start Unknown, another new problem type. These problems can be challenging because the action can be represented with subtraction, but solving the problem may involve adding. For example:

Regardless of the equation they write, students should focus on explaining how their equation matches the story problem.