Parallel structure, or parallelism, is a literary technique in which two or more phrases or sentences in a short story or a poem, for example, repeat the same grammatical structure, but with one or more words changed in the repetition of the original phrase or sentence.

The words which replace the words in the original phrase or sentence are generally the same part of speech, or words which have the same or similar sound, meaning, or meter.

In Robert Frost's poem "Mending Wall," the parallel structure is direct and uncomplicated in a simple declarative sentence like "Good fences make good neighbors" (line 26), which contrasts "good fences" with "good neighbors," and in which the sentence is itself repeated in the last line of the poem.

Another more complicated sentence, "Something there is that doesn't love a wall" (line 1), is repeated in its entirety in the poem. However, its meaning in the first line of the poem, when the first-person narrator is reflecting on the damage done to the wall by nature, is different from its meaning in line 34, when the narrator uses the sentence more abstractly—to implicate humans in the damage to the wall, and perhaps to question the existence of the wall at all.

Speaking about the "gaps" in the wall caused by nature where the wall has crumbled to the ground, and "where even two can pass abreast" (line 4), the narrator remarks, "No one has seen them made or heard them made" (line 10), an interesting parallel to the age-old question, "If a tree falls in the forest, and no one is there to hear it, does it make a sound?"

The narrator uses parallel structure to describe the unseen and unheard damage that's been done to the wall and which affects those on both sides of the wall.

To each the boulders that have fallen to each.
And some are loaves and some so nearly balls. (lines 16-17).

The narrator also uses parallelism to question the need for a wall, first as a practical matter ("Isn't it / Where there are cows? But here there are no cows" (lines 29-30)) and then more philosophically ("Before I built a wall I'd ask to know / What I was walling in or walling out" (lines 31-32)).

His neighbor says again, "Good fences make good neighbors" (line 44).