LT.xaml

  <ComboBox Name="cboLetters" ItemsSource="{Binding MainLetterList, Mode=TwoWay}" VerticalAlignment="Top" SelectionChanged="cboLetters_SelectionChanged" >
                <ComboBox.ItemTemplate>
                    <DataTemplate>
                        <TextBlock Text="{Binding Path}" />
                    </DataTemplate>
                </ComboBox.ItemTemplate>
            </ComboBox>
            
            <CollectionViewSource x:Key="SelectedLetterTemplates" IsLiveSortingRequested="True"  Source="{Binding Path=SelectedItem.Templates, ElementName=cboLetters, Mode=TwoWay}">
                <CollectionViewSource.SortDescriptions>
                    <scm:SortDescription PropertyName="IsDefault" Direction="Descending"/>
                    <scm:SortDescription PropertyName="Description" Direction="Ascending"/>
                </CollectionViewSource.SortDescriptions>
            </CollectionViewSource>
            
            <ListBox Name="lbTemplates" ItemsSource="{Binding Source={StaticResource SelectedLetterTemplates}}" Height="120"  SelectionChanged="lbTemplates_SelectionChanged">
                <ListBox.ItemTemplate>
                    <DataTemplate>
                        <StackPanel Orientation="Horizontal">
                            <Label Visibility="{Binding IsDefault, Converter={StaticResource BoolToVisibilityConverter}, ConverterParameter=false, Mode=TwoWay}" Margin="0 0 5 0" FontWeight="Bold">(Default)</Label>
                            <Label Content="{Binding Description, Converter={StaticResource SingleLineTextConverter}}"/>
                        </StackPanel>
                    </DataTemplate>
                </ListBox.ItemTemplate>
            </ListBox>