Materials - A floor mat
- A pair of yellow equilateral triangles with a black line along on of the sides.
- A pair of gray scalene right-angled triangles with a black line along the hypotenuse.
- A pair of green scalene right-angles triangles with a black line along the hypotenuse.
- A pair of yellow isosceles right-angled triangles with a black line along one of the two sides.
- A pair of green isosceles right-angled triangles with a black line along the longer side.
- A pair of yellow scalene right-angled triangles with a black line along the shorter side.
- One smaller red scalene right-angled triangle with a black line along the longer side.
- One red scalene obtuse-angled triangle with a black line along the side opposite the obtuse angle.

Presentation

Introduction
Invite the child by telling him you have something to show him. Bring him over to the correct shelves. Show the child how to carry the box. Have him carry it to the mat and have him place it of the right half of the mat. Have the child sit to your left, and then you sit down.

Constructing
- Remove the lid of the box using both hands.
- Place the lid directly in front of the box and palace the box on top of the lid.
- Take out the pair of yellow equilateral triangles, the pair of gray scalene right-angled triangles, and the pair of green scalene right-angles triangles and place them randomly to the left of the box.
- Move the box above the lid and then replace the lid on the box.
- Chose one of the green triangles and place it in isolation from the other triangles.
- Pick up the other green triangle and using the black line as your guide, match up both black lines. Rotate the two triangles if needed to clearly see the new form they have made: a square.
- Allow the child to clearly see the shape.
- Gently move the square to the top left of the table.
- In the same manner, create a rectangle using the two gray triangles and then make a parallelogram using the two yellow triangles.
- Mix all of the triangles together and allow the child to match the corresponding triangles together.

Square

Rectangle

Parallelogram

- Leave the shapes up above and remove the other triangles from the box.
- Using the black lines as your guide once again, build the remaining shapes using the matching colors and line them up under the first three shapes.

Square

Rectangle

Parallelogram

- Mix all of the triangles and have the child repeat the lesson.

Exercises
No Exercises

Language
As the child has been taught the names with the geometric cabinet,
There is no language lesson given with this material.

Purpose

Direct
To show that by joining together different triangles, 4-sided figures are formed.

Indirect
Preparation for geometry: to show that all plane figures constructed with straight lines are composed of triangles.
Preparation for understanding the concept of equivalence and its application for finding the area of place figures.

Control of Error The black lines.

Age 4 to 5 years

Video These Montessori Videos show how to present a child with the Rectangular Triangle Lesson.