Abstract: A unison gauge for indicating levelness of a string set of a grand piano is described. The gauge has a contact block adjustably engageable with the lowermost tangential aspect of the string set. Battery powered indicators provide an indication for each string. With the piano action withdrawn from the piano case, the base of the device rests on the key bed below the piano hammer strike area of the string set and holds the contact block mounted thereon on an extendable mast. The contacts of the contact block are in a plane parallel to the key bed. The contact block is under slight spring tension when engaged with a string set being monitored and, upon compression of the spring readily slides along the key bed to the next string set. A method of use of the unison gauge is also provided.