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As of this writing
- The Just Intonation Network had much information about Just Intonation, including some audio examples.
- Kyle Gann's An Introduction to Historical Tunings was a good source about both the historical background and more technical information about various tunings. It also includes some audio examples.
- The Huygens-Fokker Foundation had a very large on-line bibliography of tuning and temperament.
- Musemath had several animations illustrating equal temperament and the math necessary to understand it.