This is our original set from 2002, and is the mainstay of our tuning fork technique. We calculated the frequencies back then, gave them to our manufacturer, and have never changed them since. The set of 13 is based on the 1st audible freq. of the Schumann Earth Cavity Resonace and each pair represents the ratio of 1.5/1, the musical interval of the 5th. Pair 5 and Personal Tuners are part of this set and can be purchased separately.  The complete name is "Set of 13 Spiral of 5th Tuning Forks", 13 tuning forks - 12 pairs.