To me, the coolest part of a bicycle is the rear derailleur. It seems to be the most complicated piece, but has a relatively simple job. It can look fancy, it can look all "business" and any manner in between.

I've been fortunate enough to have been able to try out bunches of different derailleurs, and I swap them around often enough- especially on the friction bikes. Since I have a bunch, and they look so cool, why not display them when they're not being used?

I had a few coupling nuts that were just about right- I went and picked up a few more and some bolts and screwed them on to a shelf board that the bookshelf has been long gone.