A gnomon is that which when added to a shape makes a larger similar shape.

For a regular n-sided polygon (shown in white), the gnomon consists of a similar polygon (black with white outline) plus a series of emanating parallelograms (red and blue).

Use the pull-down to choose the number of sides or the Play/Pause and Faster buttons to enjoy the animation. (Fast speeds might now work well depending on how your browser caches images.)