Every Polygon and Polyline includes a FillRule property that lets you
choose between two different approaches for filling in regions. By default,
FillRule is set to EvenOdd. In order to decide whether to fill a region,
Silverlight counts the number of lines that must be crossed to reach the
outside of the shape. If this number is odd, the region is filled in; if it’s
even, the region isn’t filled. In the center area of example, you must cross
two lines to get out of the shape, so it’s not filled.

Silverlight also supports the Nonzero fill rule, which is a little trickier.
Essentially, with Nonzero, Silverlight follows the same line-counting
process as EvenOdd, but it takes into account the direction that each line
flows. If the number of lines going in one direction (say, left to right) is
equal to the number going in the opposite direction (right to left), the
region is not filled. If the difference between these two counts is not zero,
the region is filled. In the shape from the previous example, the interior
region is filled if you set the FillRule to Nonzero.

this example, the points are numbered in the order they are drawn, and
arrows show the direction in which each line is drawn.)