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In page Musical tuning:

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Harmonics may be used to facilitate tuning of strings that are not themselves tuned to the unison.[citation needed] For example, lightly touching the highest string of a cello at the middle (at a node) while bowing produces the same pitch as doing the same a third of the way down its second-highest string. The resulting unison is more easily and quickly judged than the quality of the perfect fifth between the fundamentals of the two strings.