The traditional harmonic system of Western European music is limited to ratios of prime numbers no higher than 5. The primes 2, 3 and 5 define the Major [Thirds] and Minor Thirds [with the ratios] 5/4 and 6/5 and their compounds, and name is derived from the Latin word for third, tertius.
By an accident of language, tunings limited by the prime number 5 are termed "Tertian, " while those bounded by the prime number 3 are called "Quintal," or "Quartal."
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