DockPanel
<DockPanel LastChildFill="True">
<Button DockPanel.Dock="Top" Background="AliceBlue" Content="Зверху 1" />
<Button DockPanel.Dock="Top" Background="AliceBlue" Content="Зверху 2" />
<Button DockPanel.Dock="Bottom" Background="BlanchedAlmond" Content="Знизу" />
<Button DockPanel.Dock="Left" Background="Aquamarine" Content="Зліва" />
<Button DockPanel.Dock="Right" Background="DarkGreen" Content="Справа" />
<Button Background="LightGreen" Content="Центер" />
</DockPanel>
Без Dock кнопка буде зліва і займати лише свою ширину