Just Intonation is a musical tuning system based on the natural harmonic series. It uses simple whole-number ratios to define the relationships between notes, creating intervals that sound consonant and pleasing to the ear. 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 12 equal parts. While Just Intonation can produce rich and vibrant harmonies, it can be less flexible for modulation between keys compared to Equal Temperament.