When I was in high school, I had a job with North American Rockwell designing parts for the B-1 after school. Three stiffeners are mine. Nothing more would be said. You were not supposed to ask as an airframe person. Interesting to see that all this comes back to the avionics people.
Angle brackets used in homes are stiffeners.
They fit into corners to make the structure more rigid. You make can make aircraft from a few large pieces, or from many small pieces. I am a little confused about all the recent discussion in Risks about pilot problems with autolanders, etc. I read a paper written in the early 70's about how the autolander for the L1011 was verified. So there are already autolanders in operation and have been for a long time.