Pope Gregory I (590 to 604) ordered all music used in religious celebrations to be cataloged. This spurred the creation of early musical notation and laid the foundation for classical music.
Beginning in the 11th century, one or two lines of melody, called organum, were often added to the traditional, single Gregorian-chant melody line. Organum became increasingly sophisticated over the next three centuries.
The 14th-century Ars Nova period produced an explosion of musical expression. A new notation method brought rhythmic freedom to devotional compositions, and the polyphonic sophistication of church music enlivened non-religious compositions.
Serious instrumental composition began during the Renaissance, as did the transition from modal harmonies to major and minor scales, the sound that we now associate with classical music.
The orchestra, the opera, and many new musical forms and instruments first appeared in the Baroque period (1600 to 1750). The Classical period (1750 to 1820) is noted for the standardization of sonata form, the first symphonies, and the invention of the pianoforte.