My thanks to Fred Reinagel, the inventor of the beloved VioLab tuner, for sending me his essay on double frets on the viol. I highly recommend this paper particularly for viol players as it furnishes much more detail on double frets than I was able to provide in the book. Additionally, it provides a succinct, clear, and accurate explanation of meantone temperaments with recommendations specifically geared toward consort tuning. You can access it here: Double Your Pleasure—or Double Trouble?.