Start class by reviewing the Recognizing Symmetry – Practice
activity sheet from the first lesson. Tell students to hold on to their
creations from Question #5 (generating patterns), since they will be
sharing them with a partner later on in class. However, you could
choose to have some students show off their creations to the rest of
Cyclic and Dihedral Notation
Classifying Cyclic Figures
On the web site Creating Cyclic Figures, how would you describe the figures you created? What kind of symmetry do they have?
Answer: Cyclic figures have only rotation symmetry
Pass out the Cyclic Figures – Notes page from the following activity sheet.
Go over each of the examples.
- To determine the classification, determine the number of repeating patterns.
- To determine the angles of rotation, have students draw lines
to the center from corresponding points on consecutive repeating
patterns and use a protractor to measure (see below). This angle is the
base angle; all the other angles will be multiples of this angle, up to
| ||If a figure is Cyclic n, then theformula is:
Classifying Dihedral Figures
To introduce the next section, ask students, On the website Creating Dihedral Figures, what type of figures did you create?
Answer: Dihedral figures have both reflection and rotation symmetries.
Distribute the Dihedral Figures – Notes page from the Classifying Symmetry activity sheet and go over each example.
Points to emphasize when going over this page of the activity sheet:
- The rotation symmetries are determined the same way as in classifying cyclic figures.
- The lines of reflection can always be drawn through the center
of each repeating pattern, and also between each repeating pattern.
- It is very easy for students to count each reflection line
twice after they have drawn the lines. However, the number of
reflection lines will always equal the number of rotation symmetries.
Rigid Motions of the Plane (Basic Transformations)
Ideally, you should teach this lesson after students have
discussed transformations of the plane. If that is not possible, review
the four basic rigid motions of the plane.
- Rigid motions are motions that preserve the size and shape of the figure.
- The 4 rigid motions are translations, reflections, rotations and glide reflections.
There are many good resources that demonstrate the fundamental transformations, including this complete unit from NCTM.
A frieze is a decorative horizontal strip that is made up by a repeating pattern.
To explore the 7 classes of frieze patterns, distribute the Seven Classes of Frieze Patterns – Notes page of the activity sheet and go over each pair of examples.
Additional Notes for Students:
- All frieze patterns have translation symmetry
- When a frieze pattern has vertical reflection symmetry, that
means we can draw at least 1 vertical line so that 1 side of the
pattern is a mirror image of the other side. There is often more than 1
possible vertical line.
- When a frieze pattern has horizontal symmetry, the only possible horizontal line is the line through the center of the pattern.
- The best way to determine if a frieze pattern has rotation symmetry is to look at it upside down and see if it looks the same.
- The best way to recognize glide reflection symmetry is to picture a set of footprints in the sand.
Different books and websites use different notations for the 7
classes of frieze patterns. The use of Class I, Class II, etc. is more
for ease of use than for any other reason.
Now distribute Classifying Frieze Patterns – Notes page
off the activity sheets and have students work in pairs or small groups
to fill in the blanks. Then, give the groups an opportunity to share
their answers agree. Make sure students explain their reasoning.
Compare student responses with the suggested answers to see if there is
agreement. Is it possible that different transformation sets will
produce the same image? Use the flow-chart to classify the 2 examples
on the page.
Have students exchange the results from their work at the end of the previous day's lesson Recognizing Symmetry - Practice from Recognizing Symmetry - Practice activity sheet from last class, where they used
JavaSketchpad applets to create the two cyclic figures, two dihedral figures, and two frieze patterns.
1. Assign the Classifying Symmetry - Practice from Classifying Symmetry - Practice
activity sheet. Note that for Question 6, student swaps their creations
for Question 5 from the Recognizing Symmetry-Practice assignment since
they will using them for question 6.
Assign the Classifying Symmetry – Practice page of the Classifying Symmetry — Practice activity sheet. Note that for Question 6, students exchange their results for Question 5 on the Recognizing Symmetry – Practice
activity sheet from the previous lesson, where they used JavaSketchpad
applets to create the 2 cyclic figures, 2 dihedral figures, and 2
By the end of this lesson, students will be able to:
- Recognize the symmetries present in frieze patterns.
- Classify frieze patterns into their appropriate class.