How does just intonation produce whole tones of different sizes?

In just intonation—often represented by the Ptolemaic tuning system—whole tones are produced in two distinct sizes because the system prioritizes the use of pure, acoustically natural intervals derived from the harmonic series.1

Rather than forcing all whole tones to be identical, this system utilizes two specific ratios:

• Major tones: These are tuned to a ratio of 9:8 (approximately 204 cents) and occur in intervals such as C–D, F–G, and A–B.1
• Minor tones: These are tuned to a ratio of 10:9 (approximately 182 cents) and occur in intervals such as D–E and G–A.1

This variation arises because the system is constructed by adding and subtracting pure intervals to maintain harmonic purity in specific triads. While this ensures that certain chords sound perfectly "in tune," it creates a melodic drawback where the steps between notes are not uniform, a difference that is noticeable to the ear.21