Thanks for your posts on these various tool topics.
I own a couple of folding steel rules. One, marked just "Made in Germany," unfolds to six feet. I use it pretty much only when I need a bench rule (rather than a tape measure) that opens to that length.
The other, a No. 86 by Lufkin, unfolds once, from 12 to 24 inches. This is a sheet metal worker's (aka tinbender's) rule. One face has inches, with fractions. The other face is marked in inches along one edge and circumference in inches along the other. This would be quite handy: if you were asked to make a cylinder (ductwork, round container, etc.) that was an odd dimension, like 18-1/2", you didn't need to calculate the circumference to lay out the work; just read straight off the rule, add the overlap for the seam, and proceed.
Digressing...I also own three foot and four foot non-folding steel rules for tinbenders (also, I believe, by Lufkin). These have handy tables on the back telling you, for instance, how large to make a container for a particular volume of liquid. It's been 40 years, almost, since I briefly bent tin for a living, but I can appreciate what timesavers those rules must have been in the trade.