Silverlight: HorizontalAlignment

    <Grid x:Name="LayoutRoot" Background="White">
      <StackPanel Background="Gray" Width="400">
          <TextBlock Text="HorizontalAlignment"></TextBlock>
          <Button Content="Default"></Button>
          <Button HorizontalAlignment="Center"  Content="Center"></Button>
          <Button HorizontalAlignment="Left"  Content="Left"></Button>
          <Button HorizontalAlignment="Right"  Content="Right"></Button>
          <Button HorizontalAlignment="Stretch"  Content="Stretch"></Button>
      </StackPanel>
  
  </Grid>