# How do you rotate shapes 90 degrees clockwise about the origin?

Move each point with the coordinates (a, b) to the location (b, -a).Equivalently, if the points are represented by a 2x1 column matrix, then pre-multiply by the matrix

( 0 1)

(-1 0)

### Why can only some shapes tessellate?

Shapes can tessellate only if a number of them can meet at a point and cover 360 degrees without overlap. For regular shapes this requires that the angles of the shape are a factor of 360 degrees. For non-regular shapes it is necessary that the angles of the shapes can be grouped so that they sum to 360 degrees.