Steel Straightedges Scraped True Three at a Time

A straightedge checks that a surface is flat; laid on the work against the light, or with a smear of blue, it shows the high spots. Two edges can fit each other and still both be curved, but three that all fit each other must be straight. File three steel bars, then blue, rub and scrape them in turn until every pair bears along its length. The picture: a knife-edge steel straightedge for checking flatness against the light.