An extended metaphor develops a comparison between two unlike things over several lines, stanzas, or an entire poem. Instead of making a one-time comparison, the poet keeps returning to the same metaphor and deepens it with new details, images, and associations. This can give the poem unity and help the comparison feel richer and more fully explored.
Example
The year was a harbor at dusk,
its docks full of half-tied boats;
my thoughts were gulls circling low,
my hopes were lanterns in the fog,
and every clock on shore
kept tolling like a buoy in the dark.
Example 1: Michael Chabon, The Yiddish Policeman’s Union. Harper, 2007
“It never takes longer than a few minutes, when they get together, for everyone to revert to the state of nature, like a party marooned by a shipwreck. That’s what a family is. Also the storm at sea, the ship, and the unknown shore. And the hats and the whiskey stills that you make out of bamboo and coconuts. And the fire that you light to keep away the beasts.”
Example 2: William Shakespeare, Romeo and Juliet
“But soft! What light through yonder window breaks?
It is the East, and Juliet is the sun!
Arise, fair sun, and kill the envious moon,
Who is already sick and pale with grief.”
Example 3: Martin Luther King, Jr. “I Have a Dream” speech
In a sense, we’ve come to our nation’s capital to cash a check. When the architects of our republic wrote the magnificent words of the Constitution and the Declaration of Independence, they were signing a promissory note to which every American was to fall heir. This note was a promise that all men, yes, black men as well as white men, would be guaranteed the “unalienable Rights” of “Life, Liberty and the pursuit of Happiness.” It is obvious today that America has defaulted on this promissory note, insofar as her citizens of color are concerned. Instead of honoring this sacred obligation, America has given the Negro people a bad check, a check which has come back marked “insufficient funds.”