It just says "one of the most popular", not "the most popular". The double and triple numbers are Figured Bass notation (see: http://en.wikipedia.org/wiki/Figu

It just says "one of the most popular", not "the most popular". The double and triple numbers are Figured Bass notation (see: http://en.wikipedia.org/wiki/Figured_bass http://en.wikipedia.org/wiki/Figured_bass ). So, if you have the 6th and it's the second inversion, it would be 664. For example, in G that would be the second inversion of Em.