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