Straightedge from Diderot’s Encycopedie, Plate VI, Fig. 12.
Reproduction in quarter sawn beech.
It is often astounding to me how much a small flourish of decoration, in this case an ogee, can give an object a sense of elegance. I speculated about this before, in a post about decoration on a Turkish bone folder. The decoration is pure in this case; I can think of no functional reason for it. There are, of course, several functional differences between a wood and metal straightedge that could affect how we work, such as the lightness, thickness, slight dimensional variability and non-marring qualities of wood. The material differences are important: we touch and handle wood differently than metal. Using different, or reproduction tools also jars us out of our usual, ingrained work habits. Before I made this tool, the last time I used a wooden ruler was in grade school, and it had a thin brass or tin edge. But if there is no functional difference, say between the ‘plain’ wooden ruler also depicted (Fig. 13) and this one, does the decoration change the way we use the tool–and ultimately the work we do with it?
Peachey Conservation 
@ „functional reason”— Jeff, what do you think: Are these tiny notches between straight and curved part of the edge originally identical? If (possibly) different they MAY have had a (today unknown) function.
I’d bet the notches and curve were pretty much identical, originally. It is always a possibility that they had a function: carpenter squares of the time also had similar fancy ends.
http://www.pbs.org/woodwrightsshop/video/3100/3106.html
(Thanks to Tom Conroy for the link about how to make a square)
Can you make me one? After visiting 2 Asian scroll conservation labs & seeing their wonderful wooden straight-edges, our thin metal rulers, well, they don’t “rule” do they? bd