A standard envelope is rectangular in shape. The rectangle is covered by four triangles, which are made by folding over the same piece of paper until we get two pairs of identical triangles. 

The diagonal lines in this rectangle make up the sides of the four triangles and the point where the two diagonals intersect is the point where the vertices of these four triangles meet.

But, how do we make an envelope? What shaped piece of paper do we need to use to give us a rectangular envelope when we fold over the four triangles?

Imagine what it looks like unfolded. Unfold the triangles one by one. 

Look carefully at the following window. In the window there is a closed rectangular envelope.

Esta unidad interactiva requiere la máquina virtual de Java J2RE.

1.- What shape is the original piece of paper?

2.- How is it folded to make the envelope?

3.- Why do the vertices of the rhombus find themselves in the middle of the envelope when we fold the paper along the segments joining the midpoints of the rhombus?

4.- Make an envelope 12 x 6 cm in size and describe the steps you go through to make it.

5.- Make an envelope 10 x 10 cm in size and describe the steps you go through to make it.