Muses

"The Nine Muses cranked up the tunes, and I realized the music was whatever you wanted it to be: the gods could listen to classical and the younger demigods heard hip hop or whatever, and it was all on the same sound track. No arguments. No fights to change the radio station. Just requests to crank it up."

- Percy

The Muses were daughters of Zeus, king of the gods, and Mnemosyne, Titaness of memory.

Muse usually means inspiration, either for an artist or poets, and they would pray to the muses for inspiration.

History
The Muses judged a contest between Apollo and Marsyas. They also gathered the pieces of the dead body of Orpheus, son of Calliope, and buried him. They blinded Thamyris for his hubris in challenging them to a contest.

The Muses are also the companions of the god of music, Apollo.

There are nine Muses:
 * Calliope, Muse of Heroic Poetry
 * Clio, Muse of History
 * Erato, Muse of Lyric Poetry
 * Euterpe, Muse of Music
 * Melpomene, Muse of Tragedy
 * Polyhymnia, Muse of Choral Poetry
 * Terpsichore, Muse of Dance
 * Thalia, Muse of Comedy
 * Urania, Muse of Astronomy

The Titan's Curse
The Muses played at the celebration at the end of The Titan's Curse, and tried to stop the invasion of Olympus by Kronos in The Last Olympian. They do not play major roles in the series.

In The Titan's Curse, the Muses performed at the final celebration after the gods had decided that Percy was not to be killed. According to Percy, everybody hears the music they only want to hear, like classical for the gods and hip hop for the younger demigods. He later shares a dance with Annabeth, and the song he heard changed to a slow one, though he was not sure what she was hearing.