Equal temperament is any tuning system that divides the octave into a number of equal intervals.
Equal steps like the white and black keys in each octave of the tone range. All contemporary keyboard instruments are tuned to the 12 equal temperament scale. However, except for octaves, no interval is perfectly in tune, but they are all acceptable in tune. In major-key melodies, the third and seventh degrees of the tone scale are shifted upward. The root, fourth and fifth degrees are points of melodic repose and should not be shifted.