Harmony is the simultaneous sounding of two or more notes to create chords and progressions in music. The history of harmony can be traced back to ancient times, with early forms of harmony emerging in various cultures and musical traditions around the world.

In Western music, the evolution of harmony can be divided into several distinct periods. During the Medieval period (c. 500-1400), Western music was primarily monophonic, meaning that it consisted of a single melodic line without any accompanying harmony. However, by the late Medieval period, composers began to experiment with polyphony, the simultaneous sounding of two or more independent melodic lines.

The Renaissance period (c. 1400-1600) saw further developments in polyphony and harmony, with composers such as Giovanni Pierluigi da Palestrina and William Byrd creating intricate and sophisticated harmonies in their works. The use of chords and chord progressions became more prevalent, leading to the emergence of tonality as a defining characteristic of Western music.
The Baroque period (c. 1600-1750) saw the emergence of a highly elaborate and ornamental style of music, with composers such as Johann Sebastian Bach and George Frideric Handel creating complex and highly structured works that emphasized the interplay between different musical voices and the use of counterpoint.

During the Classical period (c. 1750-1820), composers such as Wolfgang Amadeus Mozart and Ludwig van Beethoven focused on creating balanced and harmonically transparent works that emphasized clarity and simplicity. The Romantic period (c. 1820-1900) saw a return to more complex and expressive harmonic language, with composers such as Franz Schubert and Gustav Mahler using chromaticism and unconventional chord progressions to create highly emotional and dramatic works.
In the 20th century, composers such as Igor Stravinsky, Arnold Schoenberg, and John Cage pushed the boundaries of harmony even further, experimenting with atonal and dissonant music that challenged traditional notions of tonality and harmonic structure.

Today, harmony continues to evolve and change, with musicians and composers from a wide range of genres and cultures incorporating elements of traditional Western harmony as well as new and innovative harmonic techniques to create their own unique sound.