According to Donald Parry, parallelism is universally recognized as the characteristic feature of biblical Hebrew poetry. (p. i)
Apparently, the prophets and writers of the scriptures employed the repetition of alternating parallel lines for the purpose of reinforcing their teachings and doctrines. (p. x)
Antithetical Parallelism is characterized by an opposition of thoughts, or an antithesis between two lines. This "antithesis is not in terms of contradiction but in opposite aspects of the same idea. A common feature of antithetic parallelism is the article "but." (p. xxvii)
A good example is found in Ether 12:26:
Fools mock,
but they shall mourn;
[Donald W. Parry, The Book of Mormon Text Reformatted according to Parallelistic Patterns, F.A.R.M.S., p. 473]