I hope this won't cause too much gnashing of teeth because its steel work!
Having just spent 30 odd years making steel gates railings, and other fabrications, this little gadget has been with me since my first month.
Basically we have 99mm of 25 x 25 x 5mm square root angle, with two pieces of 10mm square blobbed on to it. This gives me a method of spacing measurements of 10mm, 15mm, 20mm and 25mm, from an edge, useful for finding the centers of flat bars, or hole positions for pads and brackets, without wearing out the rules on combination squares. The 99mm length is very quick check that my bar spacing on a railing does not exceed 100mm as per regs, although by using a calculator I pride myself that my bar spacing is always equal on any given panel. Similar ones have been made from other sizes of angle, so nearly every standard size measurement is covered.
Gareth