Parallelism (sometimes called thought rhyme) is a balance not only of form but also of the though between successive members in a poem. A line of poetry is divided into parts. Parallelism is more than mere repetition of words or ideas in successive parts. The second part is a specification, often an intensification, of the first. Where there is a third part, the third complements the thought of the first part.
In synonymous parallelism, the same thought is expressed in successive parts; the second part simply repeats the sense of the first in slightly different terms.
In antithetic parallelism, the thought expressed in the second part is in contrast to that of the first part.
In formal parallelism, also called synthetic parallelism, a verse contains neither repetition in different terms nor contrasted assertions. In it the thought of the first part is carried further and completed in the second.
In climactic parallelism, the characteristics of synonymous and formal parallelism are combined. The second part echoes or repeats a phrase in the part and also adds to it an element which carries forward or completes the sense.
In staircase parallelism, the second part of the verse develops the thought of the first, without quoting words from the first part.
In internal parallelism, the balance of form and thought is between individual parts of a verse. In external parallelism, there is balance not only within but also between verses.
In complete parallelism, each term in the first part is matched by a corresponding term in the second part.