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)
An "extended alternate" type of parallelism takes the form A-B-C-D/A-B-C-D. (p. xiii)
A fine example of this Extended Alternate Parallelism is found in Ether 2:8-10 and deals with the decrees of God concerning the land of promise.
a. And he had sworn in his wrath unto the brother of Jared, that whoso should possess this land
of promise,
b. from that time henceforth and forever, should serve him, the true and only God,
c. or they should be swept off
d. when the fulness of his wrath would come upon them.
a. And now, we can behold the decrees of God concerning this land, that it is a land of promise;
b. and whatsoever nation shall possess it shall serve God,
c. or they shall be swept off
d. when the fulness of his wrath shall come upon them. And the fulness of his wrath cometh
upon them when they are ripened in iniquity.
a. For behold, this is a land which is choice above all other lands;
b. wherefore he that doth possess it shall serve God
c. or shall be swept off; for it is the everlasting decree of God.
d. And it is not until the fulness of iniquity among the children of the land, that they are swept off.
[Donald W. Parry, The Book of Mormon Text Reformatted according to Parallelistic Patterns, F.A.R.M.S., p. 455]