While the Greeks typically regarded and approached their gods as individuals, there were some who could also be addressed in the aggregate–as a group. One such group was the Muses, or Mousai.
The Muses are well known to us even today, in part because over the centuries, the poets never ceased to call on them for aid; the Romantic writers were particularly fond of the goddesses. The word “muse” refers not only to the goddesses themselves and their seemingly-capricious gifting, but is as well a verb meaning mindful consideration.
The Muses, daughters of Zeus and Mnemosyne, goddess of memory (although their parentage is described differently by some sources), are spirits of inspiration, called on not only by poets and artists, but also by seekers of knowledge of all kinds. Records of their naming and number vary as well over time and region, but eventually most settled on the nine named in Hesiod’s Theogony: Calliope, Clio, Euterpe, Erato, Thalia, Melpomene, Polyhymnia, Terpsichore, Urania.
The Muses are especially companions of Apollo, who is himself a god with a great interest in poetry, music and the arts.
It was and is always appropriate to call on the Muses as a group for inspiration. It is equally appropriate to call on them as individual goddesses within their individual realms; it is not universally agreed upon which realm is assigned to which goddess, so I include here those that seem most usual.
Calliope, the eldest of the goddesses and mother of the musician Orpheus, is known as the Muse of epic poetry. Epic poetry consists of works such as Homer’s Iliad and Odyssey, and includes the much later writings of Milton and Dante, among others. Epic poetry typically features historical events and heroic characters, and is, in a word, epic. She is often depicted in art carrying writing implements.
Clio is the Muse of history. She is often depicted in art with scrolls or books.
Euterpe is known as the Muse of music and song; she is also known sometimes as the Muse of lyric poetry (a role taken according to other sources by Erato.) Lyric poetry is typically that which is personal and deals with emotional matters. Sappho is a well-known ancient lyric poet (and my all-time favorite poet!); probably the form of lyric poetry best known to us is the sonnet. Lyric poetry, as the name suggests, was at one time sung to musical accompaniment. Euterpe is often depicted in art carrying or playing a flute known as the aulos
While some sources suggest Erato as Muse of lyric poetry, others name her the Muse of love poetry, and/or erotic poetry. Since much of lyric poetry deals with matters of love, it does seem that the goddess has a legitimate interest in both areas. She is often depicted in art carrying or playing a small lyre known as the cithara.
Thalia is the Muse of comedy. She is often depicted in art carrying the comic mask.
Her counterpart, Melpomene, is the Muse of tragedy. She is often depicted in art carrying the tragic mask.
Polyhymnia is the Muse of hymns and sacred music. She is often depicted in art in a prayerful and modest pose, wearing a veil.
Terpsichore, Muse of dance, is often depicted in art carrying a lyre.
Urania is the Muse of astronomy. She is often depicted in art carrying a globe of the heavens.