Final answer:
Poets often use a conceit, an elaborate comparison that extends throughout a poem, to create extended metaphors. This device allows the poem to explore similarities between very different things deeply and comprehensively, enriching its emotional and intellectual impact.
Step-by-step explanation:
The figure of speech that is commonly used by poets to employ extended metaphors is the conceit. A conceit is an elaborate and often surprising comparison between two very unlike things. It extends throughout a poem or a section of a poem, providing a comprehensive framework for understanding the subjects being compared. In contrast, simple metaphors or similes make brief and often one-time comparisons, such as "Life is a journey" or "Eyes like stars".
An extended metaphor continues the comparison at length, developing the similarities between the two entities in various ways throughout the poem. This technique not only enriches the imagery and the emotional resonance of the poem but also allows the reader to explore the relationship between the entities in a deeper and more nuanced manner. Take, for instance, John Donne's poem 'A Valediction: Forbidding Mourning,' where the love between two people is intricately compared to a compass throughout the verses, demonstrating the concept of a conceit.