Silverlight layout of StackPanel

    <Grid x:Name="LayoutRoot" Background="White">
      <StackPanel Background="Gray" Width="400">
          <Button Content="Button"></Button>
          <Button Content="Button"></Button>
          <Button Content="Button"></Button>
          <Button Content="Button"></Button>
          <StackPanel Orientation="Horizontal" Background="LightGray">
              <Button Content="Button"></Button>
              <Button Content="Button"></Button>
              <Button Content="Button"></Button>
              <Button Content="Button"></Button>
          </StackPanel>
      </StackPanel>
  
  </Grid>