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''Equal Temperament'' is a method of tuning an instrument, so that each distinct note is equidistant and all notes share the same set of interval ratios. Thus, it could be considered that every key on an equally-tempered instrument is equally out-of-tune compared to ideal pure-tuning.
Let's go back to the example above of how moving around the circle of fifths does not end on an octave of the original note. Well, what if we slightly lessen a 5th interval from 3:2 such that we reach that 7040 HZ A8 when moving in 5ths? This would be 55 x ___^12 = 7040. Moving things around, it is ___ = (7040/55)^(1/12), which equals ~1.49831. Thus, we need to slightly flatten flat the 5th - this is called a '''''tempered'' fifth'''. (The numbers here and above are slightly off due to rounding and limiting the number of significant digits.)
Equal temperament can be based on any number of distinct non-octave tones. We are mainly concerned with 12-Tone Equal Temperament, which is how most modern guitars are designed - the 12 tones correspond to the 12 notes of the classical chromatic scale.