Specifying points the long way with Polygon.Points tag and Point
<UserControl x:Class='SilverlightApplication3.MainPage' xmlns='http://schemas.microsoft.com/winfx/2006/xaml/presentation' xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml' xmlns:d='http://schemas.microsoft.com/expression/blend/2008' xmlns:mc='http://schemas.openxmlformats.org/markup-compatibility/2006' mc:Ignorable='d' d:DesignWidth='640' d:DesignHeight='480'> <StackPanel Orientation="Horizontal"> <Polygon Fill="Red" > <Polygon.Points> <Point X="20" Y="0"/> <Point X="40" Y="40"/> <Point X="0" Y="40"/> </Polygon.Points> </Polygon> </StackPanel> </UserControl>