Read this and answer the question

The instruments currently used in most classical music were largely invented before the mid-19th century (often much earlier) and codified in the 18th and 19th centuries. They consist of the instruments found in an orchestra or in a concert band, together with several other solo instruments (such as the piano, harpsichord, and organ). The symphony orchestra is the most widely known medium for classical music and includes members of the string, woodwind, brass, and percussion families of instruments. The concert band consists of members of the woodwind, brass, and percussion families. It generally has a larger variety and amount of woodwind and brass instruments than the orchestra but does not have a string section. However, many concert bands use a double bass. The vocal practices changed a great deal over the classical period, from the single line monophonic Gregorian chant done by monks in the Medieval period to the complex, polyphonic choral works of the Renaissance and subsequent periods, which used multiple independent vocal melodies at the same time.

What practice has changed a great deal over the classical period?
vocal