Triangles that have the same basic shape, but are of a different size, are called similar triangles.

Shape is defined by angles. In other words, two triangles are called similar if all their angles are the same, but their sides are of different length.

In the diagram on the right the triangle DEC is similar to the triangle ABC. This means that these pairs of angles are equal: BAC = EDC; ABC = DEC; ACB and DCE are obviously the same angle.

Isotiles are specially designed so that you can go on making ever larger triangles but always retaining the exact same angles as one single acute Isotile or one single obtuse Isotile.

Make sure you have remembered the size of every angle (which you have already worked out), so that you will be able to check properly that you have got similar triangles, no matter how big.