As Chassis construction continues, you can see how every piece fits right into the piece next to it, precisely. Notice how the bulkhead plates hold each side plate that make up all 4 sides of the rail. Also notice how the crossmembers slide through the frame rail to precisely locate each crossmember. The crossmember tabs also slide completely through the rails to be welded on both sides of the rail and to help carry the load through the entire rail, not just one side. This also helps alleviate chassis flex.