When mass-manufacturing techniques for cutting threads (and tapping threaded holes) were invented, first for wood and then for metal, the adjustable-height stool became a solved problem.
However, those manufacturing techniques were not evenly distributed. This stool below, purportedly from the 19th century and recovered from a farm in Germany's Black Forest, is a DIY example of a height-adjustable stool. It was clearly made by someone who had access to basic woodworking tools, but no method of threading.
It appears the worn groove in the top of the lower platform accepted some type of locking cross-member, perhaps something as simple as a wedge, dowel or branch.
On the lower part of the shaft, we can see staggered holes. Presumably these were used to lock the shaft in place using a dowel on the underside of the lower platform.
We can assume the design wasn't terribly successful, as it doesn't appear to have become widespread. (I.e. have you ever seen a design like this before?)
This stool was recovered by German industrial designers Kyra Heilig and Lenn Gerlach, and documented here.
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