Equal Temperament
Equal temperament is the system of dividing the octave into 12 equal intervals, called half-steps. This is necessary because the pure notes of the overtone series do not divide the octave evenly. In the past instruments were tuned to play some keys well and others badly. With equal temperament all keys are equally out of tune. This can clearly be heard in the major thirds on a well-tuned piano.
Each note, whether white or black, is a half-step from the notes on either side of it. If there is no black note between two white notes, those notes are only a half-step apart.
The notes in the overtone series are so in tune as to be called beatless. But on a piano the only beatless intervals are the octaves. The fifths, like the interval from the 2nd to 3rd partial, on a piano can clearly be heard to beat slowly. This is caused by interference between the two waves that are not exact multiples of the same root. The thirds are more compromised and beat much faster.

If you are more curious than confused about this subject get yourself the Harvard Dictionary of Music or get to a library that has the Grove's Encyclopedia of Music. Look up temperament. You might want to take your calculator. In the meantime you can go back to the Courses menu or continue studying pitch notation.

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