School Improvement in Maryland

Using the State Curriculum: Mathematics, Grade 3

Algebra | Geometry | Measurement | Statistics | Probability | Number | Processes
Clarifications: Each clarification provides an explanation of the indicator/objective to help teachers better understand the concept. Classroom examples are often included to further illustrate the concept. While classroom examples could be shared with the students, the intended audience for the explanation/clarification is the classroom teacher-not the student. In addition, classroom examples may or may not reflect the assessment limits.

Standard 2.0 Knowledge of Geometry

Topic E. Transformations

Indicator 2. Analyze geometric figures or pictures

Objective a. Identify and describe symmetry

  • Use no more than 4 lines of symmetry

Clarification

A shape is said to have line symmetry if it can be folded on a line so that each half of the shape is a reflection of the other. The fold line, or line of symmetry, is a line of reflection where on either side lies a mirror image. Lines of symmetry can occur across a plane horizontally, vertically or diagonally. Shapes with line symmetry may have as few as one line of symmetry (see "heart" example) or as many as unlimited lines of symmetry (such as a circle).

Classroom Example

This heart has only one line of symmetry (vertically as shown). Each half is a mirror or reflected image of the other.
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The word "MOM" has one line of symmetry where on either side of the line is a mirror image of one half of the word.
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A square has 4 lines of symmetry, horizontal, vertical and two diagonals as shown here.
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A circle has limitless lines of symmetry as, when folded along it's diameter at any rotation, the two semicircles formed will always be mirror images of each other.
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In the lightning bolt there is no line where one could find an exact reflection, therefore this figure does NOT have a line of symmetry.
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A rectangle has two lines of symmetry (horizontal and vertical). A rectangle does not have diagonal lines of symmetry as each side would not be an exact reflection of the other (as demonstrated below).
/toolkit/vsc/clarification/mathematics/grade3/2E2a.xml
Resources for Objective 2.E.2.a:
CLARIFICATIONS | Public Release Items |