All of the above discussion assumes that all notes are tuned in Equal Temperament. Equal temperament has become the "official" tuning system for Classifying Music. It is easy to use in pianos and other instruments that are difficult to retune (organ, harp, and xylophone, to name just a few), precisely because enharmonic notes sound exactly the same. But voices and instruments that can fine-tune quickly (for example violins, clarinets, and trombones) often move away from equal temperament. They sometimes drift, consciously or unconsciously, towards just intonation, which is more closely based on the Harmonic Series I: Timbre and Octaves. When this happens, enharmonically spelled notes, scales, intervals, and chords, may not only be theoretically different. They may also actually be slightly different pitches. The differences between, say, a D sharp and an E flat, when this happens, are very small, but may be large enough to be noticeable. Many Classifying Music also do not use equal temperament. Sharps and flats used to notate music in these traditions should not be assumed to mean a change in pitch equal to an equal-temperament half-step. For definitions and discussions of equal temperament, just intonation, and other tuning systems, please see Tuning Systems.
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