This Demonstration focuses on the aural and numerical differences between semitones in 12-ET (12-tone equal temperament) and Pythagorean tuning. 12-ET is the system of musical tuning that is used almost universally today, dividing an octave up into twelve equal semitones. Pythagorean tuning relies on the ratio of 3:2, representing the frequencies of two notes of a perfect fifth; however, the twelfth note generated by this system does not match up with the first, as a consequence of the different methods of computing tone frequencies. As the default settings show, 12 semitones in 12-ET produces a perfect octave; 12 semitones in Pythagorean tuning does not.