What is an extended metaphor?

An extended metaphor (also called a sustained metaphor) is a metaphor that is developed over several lines or paragraphs.

The following is an example of an extended metaphor in William Shakespeare’s 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

That thou, her maid, art far more fair than she.”