Final answer:
The primary similarity between 'On Virtue' and 'Mending Wall' is their use of traditional verse forms. Both poems use structured meter, with 'Mending Wall' written in blank verse and 'On Virtue' also using a consistent metrical pattern.
Step-by-step explanation:
The main similarity between Phyllis Wheatley's poem On Virtue and Robert Frost's poem Mending Wall is that they both employ a formal structure with a consistent metrical pattern. Wheatley's poem is more explicitly about virtue and moral elevation, whereas Frost's poem is a dramatic monologue that depicts the ritual of repairing a wall between neighbors as a metaphor for maintaining boundaries in human relationships. Neither poem employs free verse; instead, they both utilize structured verse to convey their themes. Mending Wall is written in blank verse, using unrhymed iambic pentameter. On Virtue also does not use everyday language but rather a more elevated diction appropriate to its philosophical subject matter. Therefore, the main similarity lies in the use of traditional verse forms rather than free verse or everyday language.