Just intonation is a musical tuning system based on the natural harmonic series. It uses simple whole-number ratios to create intervals, which results in a more consonant and pleasing sound compared to other tuning systems. For example, the interval of a perfect fifth is represented by the ratio 3:2, while a major third is represented by 5:4. This system contrasts with equal temperament, the most common tuning system in Western music, which divides the octave into twelve equal parts. While just intonation can produce beautiful harmonies, it can be less flexible for modulation between keys, making it less practical for some musical styles.