The issues surrounding setting the stretch by ear vs machine have not been settled; machines are better at deriving the absolute placement of semitones within a given chromatic scale, whereas non-machine tuners prefer to adjust these locations preferentially due to their temptation to make intervals more sonorous. The result is that pianos tuned by ear and immediately checked with a machine tend to vary from one degree to another from the purely theoretical semitone (mathematically the 12th root of two) due to human error and perception. (If pleasing the ear is the goal of an aural tuning, then pleasing the math is the goal of a machine tuning.) This is thought to be because strings can vary somewhat from note to note and even from neighbors within a unison. This non-linearity is different from true falseness where a string creates false harmonics and is more akin to minor variations in string thickness, string sounding length or minor bridge inconsistencies.