tune a beginning pitch, and then tunes the other notes in the "temperament" using tempered interval relationships. During tuning it is common to assess fifths, fourths, thirds (both major and minor) and sixths (also major and minor), often playing the intervals in an ascending or descending pattern to hear whether an even progression of beat rates has been achieved. Once the temperament octave is finished the tuner tunes the rest of the piano, working outward from the temperament, and using octaves and other intervals to align each note with notes that have already been tuned. The following table lists theoretical beat frequencies between notes in an equal temperament octave. The top row indicates absolute frequencies of the pitches; usually only A440 is determined from an external reference. Every other number indicates the beat rate between any two tones (which share the row and column with that number) in the temperament octave. Slower beat rates can be carefully timed with a metronome, or other such device. For the thirds in the temperament octave, it is difficult to tune so many beats per second, but after setting the temperament and duplicating it one octave below, all of these beat frequencies are present at half the indicated rate in this lower octave, which are excellent for verification that the temperament is correct. One of the easiest tests of equal temperament is to play a succession of major thirds, each one a semitone higher than the last. If equal temperament has been achieved, the beat rate of these thirds should increase evenly in the temperament region. The next table indicates the pitch at which the strongest beating should occur for useful intervals. As described above, when tuning a perfect fifth, for instance, the beating can be heard not at either of the fundamental pitches of the keys played, but rather an octave and fifth (perfect twelfth) above the lower of the two keys, which is the lowest pitch at which their harmonic series overlap. Once the beating can be heard, the tuner must temper the interval either wide or narrow from a tuning that has no beatings. Stretch The tuning described by the above beating plan provides a good approximation of equal temperament across the range of the temperament octave. If extended further, however, the actual tuning of the instrument becomes increasingly inaccurate because of inharmonicity, which causes harmonics to run slightly sharp, as