Specifying points the long way with Polygon.Points tag and Point
<Window x:Class="WpfApplication1.ShapesWindow" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" Title="ShapesWindow" Height="160" Width="400"> <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> </Window>