DependencyObject/DependencyProperty

DependencyProperty's (e.g. a Canvas' left, right) are assigned to DependencyObjects, e.g. UI elements. http://blog.hackedbrain.com/2004/12/04/understanding-dependencyobject-and-dependencyproperty/

Example usage of DependencyProperty to add second ContentPresenter to a ControlTemplate

For the uncommon cases where a settable property on a DependencyObject is not a dependency property, that property will be unable to support animation, data binding, or FrameworkElement styling.

http://msdn.microsoft.com/en-us/library/cc221408(v=vs.95).aspx