Musicologist Dr. Bradley P. Lehman believes that the above symbol – which was inked by Johann Sebastian Bach just above the words “Well-Tempered Clavier” – notates Bach’s specific method of keyboard tuning which has been lost for almost 300 years.

Lehman wites: “I believe its particular sound, as an integrated part of musical practice, has profound implications…”

You can read about how the squigly line translates into precise mathematical relationships and listen to recordings of Bach’s work using the tuning at larips.com. I especially recommend reading the Informal Lecture in which Lehman does a great job of recounting his discovery and the explains historical evidence for his conclusions in a fun and easy-to-understand article.