Rhyme refers to the pattern or rhythm of sounds a poet creates in his/her poetry by using similar sounding words. A rhyme scheme refers to the recurrence of similar sounding words at the end of each line in a poem, thus creating a pattern. Rhyme schemes not only provide a rhythmical quality to the poem, but can also be used to accentuate an idea or thought or as a binding agent which creates unity in the stanzas or in the entire poem. Any deviation from the pattern would, therefore, place more emphasis on the line that is different.
When identifying a rhyme scheme, one should consider the end rhyme, i.e. the pattern of sounds repeated at the end of each line. Starting with the first letter of the alphabet, 'A,' one denotes the rhyme for the first line. Every first line will, therefore, be allocated an 'A.' One then determines whether the end rhyme of the first line is repeated and, if so, the same notation is used. After that, a line which does not repeat the same sound in its end is indicated with a 'B.' The process is repeated until the end of the poem, using the letters of the alphabet in capitalized form.
Using this method, then, one can see that the rhyme scheme in Frost's poem is ABAAB; CDCCD; EFEEF; GHGGH. Frost uses a very particular rhyme scheme that is quite original. He utilizes full rhyme throughout the poem with the exception of lines 17 and 20, where he uses half rhyme. It is obvious that the syllables at the ends of these lines do not rhyme perfectly: "hence" (line 17) and "difference" (line 20), unless one emphasizes the third syllable in "difference" and pronounces the 'e' as the first syllable in "hence."