[This is part of my How to Install Moldings series.]
Historic Example, 1928
When your chair rail pattern projects beyond the door or window trim, then the proper way of installing it is to return it on top of the casing and not butted into it.
This example I found in the historic Allen House in Birmingham, Michigan. The chair rail is a two-piece design and acts as the wainscoting cap as well, though you will often find it as a stand-alone chair rail.
Even though cutting the notch in the chair rail cap so that it fits snugly on top of the casing is very easy to do, I have seldom seen it done in any new construction.
If you are hiring a finish carpenter to install your chair rail, then make sure you let him or her know that you want yours installed this way, and not cut off blunt at the door or window casing.
And if your finish carpenter does not understand this method — and many otherwise capable finish carpenters have never been taught this method, so go easy on them — then simply point them to this post and I’m confident they’ll be able to figure it out.
Hope that helps. Have fun designing and installing your chair rail!