Simple (I think) Horizontal Line in WPF?

Creating a relatively simple data entry form, and just want to separate certain sections with a horizontal line (not unlike an HR tag in HTML) that stretches the full length of the form.

I have tried this:

<Line Stretch="Fill" Stroke="Black" X2="1"/>

Because the parent control is not a fixed width, this line causes the window to stretch to the full width of the screen.

Is there an easy way to do this without fixing the width of my parent control/window?

How about add this to your xaml:

<Separator/>

I had the same issue and eventually chose to use a Rectangle element:

<Rectangle HorizontalAlignment="Stretch" Fill="Blue" Height="4"/>

In my opinion it's somewhat easier to modify/shape than a separator. Of course the Separator is a very easy and neat solution for simple separations :)