BaselineAlignment: Center
<Page xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" HorizontalAlignment="Center" VerticalAlignment="Center"> <TextBlock Background="LightGreen"> Alignment: <Span BaselineAlignment="Center">Center</Span> </TextBlock> </Page>