The HorizontalAlignment and Vertica

The HorizontalAlignment and VerticalAlignment properties describe how a child element should be positioned within a parent element's allocated layout space. By using these properties together, layout logic for a container can position child elements within the container (either a panel or a control). Alignment properties are intended to hint the desired layout to an adaptive layout container, so basically they're set on FrameworkElement children and interpreted by another FrameworkElement container parent. Alignment values can specify whether elements align to one of the two edges of an orientation, or to the center. However, the default value for both alignment properties is Stretch. WithStretch alignment, elements fill the space they're provided in layout. Stretch is the default so that it's easier to use adaptive layout techniques in the cases where there is no explicit measurement or no DesiredSize value that came from the measure pass of layout. With this default, there's no risk of an explicit height/width not fitting within the container and being clipped until you size each container.