Gradually, a cappella singing began to spread beyond the walls of monasteries and churches. In the Renaissance period, a cappella polyphony (multiple melodies sung at the same time) began to develop in Europe. This new style of a cappella singing was used in secular music as well as religious music, and it became very popular in the upper classes.
In the Baroque period, a cappella singing continued to flourish in Europe. Some of the most famous composers of a cappella music from this period include Giovanni Pierluigi da Palestrina, Tomás Luis de Victoria, and William Byrd.
In the 19th century, a cappella singing began to decline in popularity in Europe, but it continued to thrive in the United States. In the early 20th century, a number of new a cappella groups emerged in the United States, such as The Revelers, The Four Freshmen, and The Mills Brothers. These groups helped to popularize a cappella music and bring it to a wider audience.
In the 21st century, a cappella music is still popular and is performed by a wide variety of groups around the world. There are a number of a cappella festivals and competitions held each year, and a cappella music is often featured in movies and television shows.