Tips for Teaching How to Classify a Triangle
Classifying triangles comes down to two questions: what are the angles, and what are the sides? Kids mix up the six names more than the math itself.
I taught 3rd to 5th grade math for 13 years, and these are the memory tricks, examples, and games I’d use. Starting triangles for the first time, or reviewing before a test? Both work here.
How do you classify a triangle? Look at it two ways and give it one name from each.
- By angles: acute (all angles less than 90°), right (one 90° angle), or obtuse (one angle greater than 90°).
- By sides: equilateral (3 equal sides), isosceles (2 equal sides), or scalene (no equal sides).
So a triangle with a 90° angle and two equal sides is a right isosceles triangle.
How do kids remember the types of triangles?
One of the trickiest parts of classifying triangles for some students may be knowing the definitions of the different terms. Students can easily confuse new terms like obtuse, isosceles, and scalene, so it’s best to help them think of ways to remember what each term means. These word associations can help students quickly remember the different terms and help them better understand how to classify a triangle.
| Type | Sorted by | Rule | Memory trick |
|---|---|---|---|
| Acute | Angles | All 3 angles less than 90° | “A-cute little triangle” |
| Right | Angles | One angle is exactly 90° | Make an L with your right hand |
| Obtuse | Angles | One angle greater than 90° | “Obtuse = open” arms |
| Equilateral | Sides | All 3 sides equal | “Equal” is hiding in the word |
| Isosceles | Sides | 2 sides equal | Underline the 2 S’s |
| Scalene | Sides | No sides equal | The odd one out |
Classifying Triangles by Angles
In an acute triangle, all three angles are less than 90°. Since the angles are all “smaller” than 90°, you can suggest students think of this type of triangle as “a-cute little triangle”.
Obtuse triangles have one angle that is greater than 90°. When looking at an obtuse triangle, you can picture someone with open arms on one side of the triangle at the obtuse angle. Students can think of “obtuse-open” to help them remember this triangle type.
You can even go as far as saying “obtuse” in a loud, deep voice. You can even hold your arms out in a wide-angle as you say the word. This is a silly way to encourage students to make that connection that obtuse triangles are a large angle.
Right-angle triangles might be the easiest to remember if students are familiar with right angles. Holding up their right pointer finger and thumb to make a right angle or an L and then using their left pointer finger to complete the triangle is a great way to demonstrate a right triangle.
Classifying Triangles by Sides
Equilateral triangles have three equal sides. This term in itself should help students remember that this triangle is completely equal on all sides. When you introduce equilateral triangles, ask students to find a word inside that looks familiar to them. Just another way to help students make that connection that all three sides are equal.
Isosceles triangles have two equal sides. Encourage students to think of having two eyes and the word isosceles starts with the letter i. The word isosceles also has the two Ss in the front half of the word. Every time students write the word isosceles, have them underline the two Ss. This is a great way to remember that isosceles = 2 equal sides.
Scalene triangles might be the most difficult triangle to associate with anything. Since it’s the oddball term without any easy trick, students can think of scalene triangles as the odd one out: three different side lengths and three different angles.
How do you classify a triangle by sides and angles?
Have kids name the angle type first, then the side type. Every triangle gets two names.
- Angles of 60°, 60°, 60° with 3 equal sides: acute equilateral
- Angles of 90°, 45°, 45° with 2 equal sides: right isosceles
- Angles of 120°, 30°, 30° with 2 equal sides: obtuse isosceles
- Sides of 3 cm, 4 cm, and 5 cm with a 90° angle: right scalene
- Angles of 50°, 60°, 70° with no equal sides: acute scalene
A quick check kids can use: the three angles always add up to 180°. If their numbers don’t, something’s off.
In 4th grade, the standard asks students to recognize and identify right triangles and classify shapes by their angles. In 5th grade, they sort shapes into categories and subcategories, so this is a good time to ask “can a triangle be both right and isosceles?”
What mistakes do students make classifying triangles?
- Only giving one name. Ask for the angle name and the side name every time.
- Thinking a triangle can have two right angles. Two 90° angles already add to 180°, so there’s nothing left for the third angle. Same for two obtuse angles.
- Guessing side lengths by eye. Teach them to look for tick marks or measure with a ruler.
- Missing a right triangle that’s turned. Rotate the paper. A right angle is still a right angle when it’s not sitting at the bottom.
- Equilateral vs isosceles. Some curriculums count an equilateral triangle as a special isosceles triangle (it has at least 2 equal sides). Check which definition your program uses and teach that one.
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Once students have learned the different terms and definitions relating to classifying triangles, they will need lots of repetition and hands-on practice to master classifying triangles on their own. Here are some ways you can support them in the classroom with opportunities to practice identifying and classifying triangles.
Using Songs and Videos to Classify Triangles
Flocabulary has a fantastic music video on classifying triangles. This video walks through classifying triangles by the length of their sides and by the size of their angles. The visuals in this video are great and there are also some fun body motions that you can have your students do along with the video!
Encourage your students to stand during the video and do the motions along with the video to demonstrate acute, right, obtuse, isosceles, scalene, and equilateral triangles. (The motions are at the 1:50 and 3:10 marks in the video.)
The physical movements will offer a brain break and help students retain what they’re hearing in the video. You may even find them (or yourself) humming or singing the song throughout the day!
Play Games and Get Hands-On
Some students may pick up on the types of triangles through lectures or worksheets. Other students may need a more kinesthetic approach to help them learn to classify triangles. I’ve created several triangle hands-on activities and games that are perfect to use in the classroom for math centers, partner work, or just as a way to offer extra practice. These work great as a station. Here’s how I set up 5th grade math stations.
Triangle Board & Matching Games
You can never have too many board games in the classroom. Board games help students stay engaged with the concept they’re learning but also give them a bit of social skills practice as well. Partnering students with varying academic and social levels will allow them to learn from, coach, and support each other.
Here are a few of my favorite board games to help students practice classifying triangles. Want kids to design their own? Here’s how to use student-created board games.
Classify Triangles Math Board Game
This triangle board game is a great way to slip in some extra practice and test students’ knowledge of triangles. Students take turns moving around the board to different triangle spaces. Everyone playing will identify the type of triangle on the board and then students compare their answers.
This game will prompt great conversations between students when they discuss their answers. Since all students answer each problem there are plenty of opportunities for them to provide supporting facts when their answers may differ from their peers’.
Triangles Matching Game
Another great way to test your students’ knowledge while giving them extra practice is this triangles matching game. This game is great for individual practice or can be used with a partner or small group.
Students match names of different types of triangles with descriptions or pictures. Just like the board game, students can use this as an opportunity to provide their reasoning for choosing a specific answer. Peer discussion like this may even help some students better understand the concepts.
Classifying Quadrilaterals & Triangles Mobile
This quadrilaterals and triangles mobile can be used in so many ways and is a great way for students to get “hands-on” practice with shapes. Included in this resource are quadrilaterals and triangles, so it’s great to have on hand in any math classroom.
Students can either create mobiles, posters or simply use this resource as a sorting activity. You can also use the pages for interactive notebooks or study aids.
Want triangle games ready to go?
The triangle board game and matching game in this post are part of the Sneaky Learning Club, my math activity membership for 3rd to 5th grade. You get games and activities for the whole year, so you’re not hunting for a new one every unit.
Same math. Different feel.
Next up: teaching how to classify quadrilaterals.
Frequently Asked Questions
Acute, right, and obtuse (sorted by angles), and equilateral, isosceles, and scalene (sorted by sides). Every triangle gets one name from each group.
Yes. A triangle with a 90° angle and two 45° angles has two equal sides, so it’s a right isosceles triangle.
No. The angles in a triangle add up to 180°, and two right angles already use all 180°.
No. All three angles in an equilateral triangle are 60°, so it’s always acute.
Usually 4th grade, where students identify right triangles and classify shapes by angles. 5th grade builds on it by sorting shapes into categories.
Tomorrow, draw one triangle on the board and ask for both names. You’ll see right away who’s got it.
