Equal temperament is a system of tuning in which every pair of adjacent notes has an identical frequency ratio.
In equal temperament tunings an octave is divided into a series of equal "steps" i.e. equal frequency ratios, ascending in a geometric progression.
It is the present standard method of dividing the any octave of frequencies. Thus, the 12 steps we get are the 12 notes - A A# B C C# D D# E F F# G and G#, the following A being the 1st note of the next octave (double in frequency of the previous lower A).
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