lengthwise framing members (stringers and longerons) between vertical framing members(formers, frames, and bulkheads)
See:
https://books.google.com/books?id=OaXyBWO0qMkC&pg=SA2-PA54&l...

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Based on the discussion below:

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Frame: always open. As mentioned, Boeing is big on frames, though they're not super-consistent in it. The Boeing frame doesn't actually touch the skin. The frame holds a collection of "shear-ties" (that do) and "stringer clips" that unsurprisingly clip into stringers.

The key bit of the fuselage frame is that it holds one or both of these types of structure in relative position. The frame, sheartie, and stringerclip buildup create a generic 'rib' that allows for the pressurization cycle by not creating any hardpoints.

Bulkhead: it can look almost exactly like a frame in that it can be open or not. The key difference is that stringer loads terminate on a bulkhead. Usually through fittings, and usually another set of fittings out the other side -- sometimes through a mating bulkhead. This is where sections meet or large structure comes to bear. As such, it almost always touches or defines the skin loft. The Frame sees no hat-section stringer loads aside what the stringerclips might introduce, the Bulkhead sees it all.

Former: This term is getting more 'airplay' than it once did. A Former most-often applies to direct attach to skin without Bulkhead load termination. What was once generally called a "rib". It generally does not tie to the stringers -- although sometimes it's clipped in. I think this is something the pressurized planes wanted to escape the Rib-word winky smile

Rib: Almost always ties to skin and usually clips to stringers when present. Usually, in an unpressurized plane, the attachment to skin and stringer is not a fatigue concern so a more generic term like 'rib' is often used.

Obviously, there are no hard and fast rules on this. Just some kind of legacy trends over the years.

Rule 1: when at Boeing, don't call a Frame a Rib. They love to catch people doing this.

http://www.eng-tips.com/viewthread.cfm?qid=296636