Truing

Truing brings a surface, edge, or tool back into genuine flatness, straightness, or squareness -- re-flattening a sharpening stone that's worn into a dish shape, jointing an edge that's crept slightly out of straight, or squaring up a reference face that's drifted from true. The distinction from simply "flattening" is subtle but real: truing implies correcting something that has drifted out of an accuracy it's supposed to hold, a maintenance and correction task, rather than establishing flatness on a rough piece of stock for the first time.