This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
The nature of this tool is expressed in its name. It consists of a long (5 or 6 ft.) strip of well-seasoned wood or of bright hardened steel (nickel-plated if preferred), several inches wide, having at least one edge perfectly level and true throughout. Its use is for ascertaining whether a surface is uniformly even, which is readily done by simply laying the straight-edge on the surface, when irregularities of the surface become apparent by spaces between the two planes in contact. Steel straight-edges are made with one bevelled edge and with English or French scales graduated on them.