The Underlevel/String Height Gauge calibrates the leveling of strings from underneath, where the hammers strike. Level indexes the horizontal of the keybed. A light-gauge spring (height-adjustable from underneath) lifts the plunger to touch the lowest string(s). With a stringing hook, lift the ones that zing when plucked until all unison strings zing. The Underlevel/String Height Gauge also measures and records grand string heights at strike.  It will record up to 10 string heights on its aluminum plunger through vertically distinct registration holes (see String Height Gauge Assembly info).