While this literary device is used by many a writer, metaphor is one of the most useful literary tools of poets, to be sure. Certainly, many poems contain a controlling metaphor that significantly conveys its meaning. The identification of this controlling metaphor is what helps the reader/listener "break the code" of the poem. In Wallace Stevens "Sunday Morning," for example, the unstated comparison of Jove with Christ juxtaposes the love of nature with the practice of Christianity. In the poem "The Road Not Taken" by Robert Frost the controlling metaphor about making choices has meaning that deepens throughout the poem.
In figurative writing metaphors may be in one of four forms. According to Perrine's Literature: Structure, Sound, & Sense, these forms are as follows:
1. In this form of metaphor, as in simile, both the literal and figurative terms are named, e.g. "the milk of human kindness."
2. In this form of metaphor, the literal term is named, but the figurative term is implied. e.g. In Langston Hughes's poem, "Harlem," the question is asked, "What happens to a dream deferred?" Here, the literal term is mentioned. But, the figurative term to which "dream" is compared, bomb, is only implied in the last line, "Or does it explode?"
3. In this form of metaphor, the literal term is implied, but the figurative term is named. e.g. In Emily Dickinson's poem, "It Sifts from Leaden Sieves," in the line "It fills with Alabaster Wool," the figurative term is named: "Alabaster Wool." But, the literal term is implied in "It" (snow).
4. In this form of metaphor, the literal term is implied, and the figurative term is also implied. e.g. Emily Dickinson's poem, "It Sifts from Leaden Sieves," the literal term is only referred to as "It." But, as the reader continues through the poem, he/she infers that "it" is snow, which is never named.
It makes an Even Face
Of Mountain, and of Plain--
The figurative term is also implied. In line one, for instance, "It sifts from Leaden Sieves," Leaden Sieves is the figurative term for clouds.
Source: Arp, Thomas R. and Johnson, Greg. Perrine's Literature: Structure, Sound and Sense. Boston: Wadsworth Cengage Learning, 2009. Print.