In addition to the Bible and Greek and Roman mythology, poems most often allude to other poems. Poets frequently allude to other poets as a way of demonstrating their appreciation of poetry and of paying their predecessors respect. These allusions are a way to express gratitude to someone the poet considers influential. In some cases, however, allusion serves a reverse function. In satirical works, authors allude to poets they wish to take down a peg. Philip Larkin and Billy Collins allude to John Dryden and Emily Dickinson, respectively, but make irreverent sexual remarks to shake the giants of their pedestals.
Poets introduce themselves through allusion. With satire, poets position themselves as equal, if not superior, to an esteemed colleague. Even expressing admiration for another writer is a little self-serving. The praise, while sincere, gives a flattering picture of the poet's education and taste. It can even imply that the poet is going to fill the predecessor's shoes. Some poets use allusion to assert not only their own worth, but the worth of an entire group. Allusion allows poets who win mainstream recognition despite prejudice to celebrate, rather than abandon, their heritage. W.B. Yeats' poetry references Irish legend, and Duriel E. Harris incorporates African-American blues.
Allusion also has a political affect when poets apply it in poems about current events. One example is Alfred Lord Tennyson's poem, "The Charge of the Light Brigade." The poem describes a disastrous episode of the Crimean War, in which poor communication among officers launches 600 soldiers into a doomed attack. Tennyson uses "Valley of Death" to repeatedly allude to Psalm 23: "Yea, though I walk through the valley of the shadow of death, I will fear no evil: for thou art with me." Associating the valley of Balaclava with the valley of the Bible makes the brigade's military failure a triumph of faith. Thanks to the power of allusion, what might have been a national embarrassment becomes a point of pride.
Allusions to Greek and Roman myths frequently appear in poetry. The allusions work in part as a kind of shorthand -- simply by dropping a name, the poet provides a whole backstory. Familiarity with classical literature has wained, so names like Endymion, Hyperion, Pieria and Heliades strike many 21st-century readers as more confusing than informative. To appreciate the affect of allusion, consider a more recognizable example: Odysseus, also known as Ulysses. Tennyson's poem "Ulysses" is a moving portrait of an adventurous spirit and an aging king who unwilling to retire from discovery and glory. Tennyson needs the background of "The Odyssey" to give his verse weight, to appeal to readers' investment in the character. Paraphrasing the whole of "The Odyssey" would leave little room for a detailed, introspective sketch, but simply alluding to "The Odyssey" does.