Mullions

I’d cut recesses into the frame pieces to receive the mullions. And, if the mullions are to be flush or nearly so with the front of the frames, notch the ends of the mullions, too.

-- Quantum materiae materietur marmota monax si marmota monax materiam possit materiari?

You can cut a mdf square as a spacers for how far down on the stiles and how far in on the rails you would like the mullions. Use a router with a quide bushing and a bit the width of your desired mullion. Then simply pass your router into the frame sides. The depth of the cut will determine the thickness of the mullion. Put in your mullion stock and where it overlaps, cut your half lap joint. Changing the size of the mdf square will adjust where the mullions will land inside the frame.

-- http://www.efcabineture.com/ "Experience is what you get, when you don't get what you want"