Triangles are similar if they have the same shape, but not necessarily the same size. You can think of it as "zooming in" or out making the triangle bigger or smaller, but keeping its basic shape. In the figure above, as you drag any vertex on triangle PQR, the other triangle changes to be the same shape, but half the size. In formal notation we can write
which is read as "Triangle PQR is similar to triangle P'Q'R' ". The letter with a small vertical dash after it such as P' is read as "P prime".