One of my favorite and well used measuring devices is a transparent Graphic Smarts ruler. This ruler is one of a set of two. The ruler pictured here is from the second set I purchased. The first set wore out and I was lucky to find another several decades ago. I don’t think this particular set is currently being made.
They were designed to be used as graphic design rulers which included measures in points as well as inches. One of the rulers had type sizes that you could lay over type samples to get point sizes. This particular ruler has metric on one side and inches on the other and includes some useful tools for converting inch fractions to decimal and metric, rule/stroke weight gauge, and Macintosh Key Caps. It is flexible and transparent although it is has yellowed a bit with age. It was made by the C-Thru Ruler Company Bloomfield Ct.
I use the rulers mostly for creating art work these days instead of graphic design work which is why I bought them in the first place.
I found what looks like an equivalent set on Amazon made by the C-THRU called Accu Spec II.
Metric and inches and Macintosh Key Caps useful if you are working withy a Mac.
Handy conversion table for converting fractions of inches to decimal and metric and rule/stroke gauges.