An explanation is here (in interlingua).

Un explanation es hic.

# Zarlino-like intervals (as separate prime factors in numerator
# and denominator) for 12 semitones in the octave.
#
# The program zarlino will then calculate the ratios
# for all combinations of pitches.
      1 : 1         # unison, prime  1:1
2 2 2 2 : 3 5       # minor second  16:15
    3 3 : 2 2 2     # major second   9:8
    2 3 : 5         # minor third    6:5
      5 : 2 2       # major third    5:4
    2 2 : 3         # fourth         4:3
  3 3 5 : 2 2 2 2 2 # augmented fourth 45:32
      3 : 2         # fifth          3:2
  2 2 2 : 5         # minor sixth    8:5
      5 : 3         # major sixth    5:3
    3 3 : 5         # minor seventh  9:5, or 16:9
    5 3 : 2 2 2     # major seventh 15:8
#     2 : 1         # octave         2:1