The bulkheads run the width of the plane at about every 2 feet or so. They provide the skeleton for the skin to attach, as well as a framework for other items to attach. 

Here you can see all the bulkheads. Firewall is at the garage door end. A string lets me know everything is in line front to back.

Here the longerons are on place. These are 14ft long single pieces of aluminum angle with compound curves in them. They run the length of the fuse and lay right onto the jig.

Here is the 606 bulkhead with the elevator rod hole cut out. This one sits directly behind the baggage compartment. Also the elevator bellcrank attached here, as well as some other items.

Here is the 607 bulkhead. Carpenters squares, plumb bobs, and clamps help me line er up. Oh and this is upside down here. I Iater realized I had a few of these upside down and fixed it. The holes for the rudder cables go on up when in the jig. (remember the fuse is mounted upside down in the jig, so these holes will be on the bottom when the fuse is sitting right side up.)

Here is the F611 and F612 bulkheads in the rear. You can see the clamps holding the longerons to the jog. Also a notch in the vertical 2x4 had to be made to accommodate the longerons in the F612.