Congruence in Two Dimensions

Students identify, perform, and algebraically describe rigid motions to establish congruence of two dimensional polygons, including triangles, and develop congruence criteria for triangles.


Unit 2

10th Grade

Unit Summary

In Unit 2, students build off the work done in Unit 1 to use rigid motions to establish congruence of two dimensional polygons, including triangles. Through a transformational lens, students develop congruence criteria for triangles that serve as the foundation for establishing relationships for right triangles and trigonometry in Unit 4. In this unit, students are also introduced to relationships with parallelograms, and this serves as a foundation for further polygon relationships in Unit 5

In this unit, students will identify, perform, and describe transformations algebraically, using constructions both on and off the coordinate plane to justify congruence. Students will describe how the constructions used to perform the rigid motions point to the properties of the transformations, and, in some cases, indicate other transformations that are equivalent. In 8th grade, students worked with congruence through transformational geometry and learned about the interior angle sum theorem for triangles as well as the relationship between interior and exterior angles of a triangle. This understanding is formalized in this unit.

Students will learn about the criteria needed for triangle congruence, building from 7th grade, where students learned to identify conditions that determine a unique triangle, more than one triangle, or no triangle. Students will also learn how and why these criteria exist by exploring the transformations underlying the criteria.

Because triangles make up most other polygons, examining and proving theorems about triangle congruence will help students develop the foundation needed to understand the properties and criteria for congruence of other polygons. The work of this unit is also particularly important preparation for trigonometry because trigonometry is based on triangle relationships. Developing an understanding of triangle congruence will ease students’ transition to trigonometry.

Pacing: 20 instructional days (18 lessons, 1 flex day, 1 assessment day)


The following assessments accompany Unit 2.


Use the resources below to assess student understanding of the unit content and action plan for future units.

Unit Prep

Intellectual Prep

Internalization of Standards via the Unit Assessment

  • Take unit assessment. Annotate for: 
    • Standards that align to each question 
    • Purpose of each question: spiral, foundational, mastery, developing
    • Strategies and representations used in daily lessons
    • Relationship to Essential Understandings of unit 
    • Lesson(s) to which the assessment points

Internalization of Trajectory of Unit

  • Read and annotate “Unit Summary."
  • Notice the progression of concepts through the unit using “Unit at a Glance”
  • Do all target tasks. Annotate the target tasks for: 
    • Essential understandings
    • Connection to assessment questions 

Essential Understandings

  • Rigid motions can be performed to map one figure to another in single or a composition of transformations. These transformations can be performed using constructions, patty paper, and on a coordinate plane using properties of perpendicular and parallel lines.
  • Congruence can be determined through identification of rigid motions that will map one figure onto another. Because rigid motions are angle and distance preserving, corresponding parts of transformed figures through rigid motions are congruent.
  • Geometric facts are established through proof by determining a sequence of logical statements and reasons that drive towards a conclusion that is the theorem proved. Through this process, we can establish new facts.
  • Triangle congruence can be proven using side and angle relationships between corresponding parts as well as rigid motions.


polygon parallel/perpendicular interior angles exterior angles
parallelogram convex/concave congruence Perpendicular Bisector Theorem
Hypotenuse Leg Congruence Criteria property of opposite angles in parallelograms Rotational Symmetry Reflectional Symmetry
Side Angle Side Congruence Criteria Angle Side Angle Congruence Criteria Side Side Side Congruence Criteria
Angle Angle Side Congruence Criteria Base Angle of Isoceles Triangles Theorem property of opposite sides in parallelograms properties of diagonals in parallelograms


  • compass
  • straightedge

Lesson Map

Topic A: Introduction to Polygons

Topic B: Rigid Motion Congruence of Two-Dimensional Figures

Topic C: Triangle Congruence

Topic D: Parallelogram Properties from Triangle Congruence

Common Core Standards


Major Cluster

Supporting Cluster

Additional Cluster

Core Standards


  • G.CO.A.1 — Know precise definitions of angle, circle, perpendicular line, parallel line, and line segment, based on the undefined notions of point, line, distance along a line, and distance around a circular arc.
  • G.CO.A.2 — Represent transformations in the plane using, e.g., transparencies and geometry software; describe transformations as functions that take points in the plane as inputs and give other points as outputs. Compare transformations that preserve distance and angle to those that do not (e.g., translation versus horizontal stretch).
  • G.CO.A.3 — Given a rectangle, parallelogram, trapezoid, or regular polygon, describe the rotations and reflections that carry it onto itself.
  • G.CO.A.4 — Develop definitions of rotations, reflections, and translations in terms of angles, circles, perpendicular lines, parallel lines, and line segments.
  • G.CO.A.5 — Given a geometric figure and a rotation, reflection, or translation, draw the transformed figure using, e.g., graph paper, tracing paper, or geometry software. Specify a sequence of transformations that will carry a given figure onto another.
  • G.CO.B.6 — Use geometric descriptions of rigid motions to transform figures and to predict the effect of a given rigid motion on a given figure; given two figures, use the definition of congruence in terms of rigid motions to decide if they are congruent.
  • G.CO.B.7 — Use the definition of congruence in terms of rigid motions to show that two triangles are congruent if and only if corresponding pairs of sides and corresponding pairs of angles are congruent.
  • G.CO.B.8 — Explain how the criteria for triangle congruence (ASA, SAS, and SSS) follow from the definition of congruence in terms of rigid motions.
  • G.CO.C.10 — Prove theorems about triangles. Theorems include: measures of interior angles of a triangle sum to 180°; base angles of isosceles triangles are congruent; the segment joining midpoints of two sides of a triangle is parallel to the third side and half the length; the medians of a triangle meet at a point.
  • G.CO.C.11 — Prove theorems about parallelograms. Theorems include: opposite sides are congruent, opposite angles are congruent, the diagonals of a parallelogram bisect each other, and conversely, rectangles are parallelograms with congruent diagonals.
  • G.CO.C.9 — Prove theorems about lines and angles. Theorems include: vertical angles are congruent; when a transversal crosses parallel lines, alternate interior angles are congruent and corresponding angles are congruent; points on a perpendicular bisector of a line segment are exactly those equidistant from the segment's endpoints.

Similarity, Right Triangles, and Trigonometry

  • G.SRT.B.5 — Use congruence and similarity criteria for triangles to solve problems and to prove relationships in geometric figures.

Foundational Standards


  • 7.G.A.2
  • 8.G.A.1
  • 8.G.A.2
  • 8.G.A.3
  • 8.G.A.5
  • 8.G.B.6

Future Standards

Similarity, Right Triangles, and Trigonometry

  • G.SRT.A.1
  • G.SRT.A.2
  • G.SRT.A.3
  • G.SRT.B.4
  • G.SRT.B.5
  • G.SRT.C.6

Standards for Mathematical Practice

  • CCSS.MATH.PRACTICE.MP1 — Make sense of problems and persevere in solving them.

  • CCSS.MATH.PRACTICE.MP2 — Reason abstractly and quantitatively.

  • CCSS.MATH.PRACTICE.MP3 — Construct viable arguments and critique the reasoning of others.

  • CCSS.MATH.PRACTICE.MP4 — Model with mathematics.

  • CCSS.MATH.PRACTICE.MP5 — Use appropriate tools strategically.

  • CCSS.MATH.PRACTICE.MP6 — Attend to precision.

  • CCSS.MATH.PRACTICE.MP7 — Look for and make use of structure.

  • CCSS.MATH.PRACTICE.MP8 — Look for and express regularity in repeated reasoning.

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Unit 1

Constructions, Proof, and Rigid Motion


Unit 3

Dilations and Similarity

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