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Grade 4 · Mathematics · 25 minutes

Lines of symmetry

Fold a paper square so the two sides match exactly. A symmetry line divides a figure into matching reflected halves.

Mr. S Teaches · Starter mini-lesson · elem-4-lines-of-symmetry

Learning objective

Explain and practice lines of symmetry using the activity and evidence, then show the reasoning behind an answer.

Gather your materials

  • Paper and pencil
  • Counters, blocks or paper pieces
  • A ruler, clock model or grid when named in the steps

Use the items named in the teaching steps; paper models or adult-prepared photographs can replace physical equipment.

Common Core skill focus

4.G.A.3

Identify and draw fold lines that produce matching halves.

Read the official standard ↗

Code and grade verified 2026-11-03. This mini-lesson addresses a selected skill, not the entire standard.

Teach it step by step

  1. Fold a paper square so the two sides match exactly.
  2. Mark the fold as a reflection line.
  3. Try vertical, horizontal and diagonal folds.
  4. Check that a proposed fold matches every point on both halves.

Watch, notice, discuss

Video choices for this lesson

2 choices

Preview a video, then choose what fits your learners. “Show to class” opens a larger view; use the player’s play and full-screen controls when you’re ready.

Teacher preview needed. Check the language, level and length before showing. YouTube may show ads or related videos.

After watching

Draw a possible symmetry line and test whether the halves match when reflected.

An external player loads only after you choose Preview or Show to class. Playback connects to YouTube. Videos belong to their creators; availability and embedding can change.

Make the thinking visible

Example to work through together

A square has 4 symmetry lines: vertical, horizontal and both diagonals.

Try the starter activity

Fold a square so the halves match; try several folds.

Student practice

Test a nonsquare rectangle and an equilateral triangle with folds or tracing.

Quick check for understanding

How many symmetry lines does a square have?

Sample answer and teacher guidance

Sample answer: 4

Listen or look for: A nonsquare rectangle has 2 lines; an equilateral triangle has 3. Orientation does not change the counts.

Invite the learner to explain, demonstrate, draw or dictate their reasoning. Revisit a teaching step when the explanation is unclear.

Make it work for your learner

A little more support

Start with a folded square and trace corresponding corners.

An additional challenge

Find a quadrilateral with exactly one symmetry line and justify it.