Many possible aids in making absolute pitch judgments are enumerated by Helmholtz and Abraham. They are (1) feelings of tension in humming, (2) emotional reaction, (3) pain sensations caused by high notes, effects of the characteristic pitch of the ear drum on g4 and on notes which possess g4 as one of their overtones, and (4) the quality or intensity of tone as a result of the method of striking the black and white keys. In judging keys on the piano the effects of constant errors in tuning, and in judging keys on bowed and wind instruments the variation in intonation of the fifths might serve as secondary criteria.