Brad McCoy defines chiasm as "the use of inverted parallelism of form and/or content which moves toward and away from a strategic central component" and says:
In its most general sense, chiasmus involves inverted parallelism
between two or more (synonymously or antithetically) corresponding
words, phrases, or units of thought1
John Breck states it is important to distinguish between chiastic form and the authentic chiasmus:
For an authentic chiasmus produces balanced statements, in direct, inverted, or antithetical parallelism, constructed symmetrically about a central idea.2
The chiastic form may aid in memorization, but the primary purpose is to highlight the most important idea the writer intends to convey:
By detecting genuine chiastic patterns, as distinct from various parallel arrangements, we discover the “pivot” or central theme about which the author has developed other related elements in concentric symmetry. By failing to observe and appreciate that symmetry, we tend to misread the “conceptual center” and consequently to distort the author’s message. But we also miss the sense of balance and intensity that chiasmus provides. While it original purpose seems to have been mnemonic, facilitating memorization through repetition of ideas around a central theme, its chief merit is to convey poetic beauty and theological meaning through literary forms.3
R. Alan Culpepper gives an example of John's Prologue:4
A: The Word as theos with God (1-2)
B: Creation came through the Word (3)
C: We have received life from the Word (4-5)
D: John the Baptist was sent to testify (6-8)
E: Incarnation and the response of the world (9-10)
F: The Word and His own (Israel) (11)
G: Those who accept the Word (12a)
X: He gave authority to become children of God (12b)
G': Those who believe the Word (12c)
F': The Word and His own (believers) (13)
E': Incarnation and response of the community (14)
D': John the Baptist's testimony (15)
C': We have received grace from the Word (16)
B': Grace and truth came through the Word (17)
A': The Only Begotten theos with the Father (18)
The key statement is believers are children of God. The seven pairs of supporting statements function primarily to surround this statement in facts rather then aiding in memorization.
Notes:
1. Brad McCoy, Chiasmus: An Important Structural Device Commonly Found in Biblical Literature. p 18-19 [Chafer Theological Seminary]
2. John Breck, The Shape of biblical Literature, St. Vladimir's Seminary Press, 1994, p. 18
3. John Breck, Biblical Chiasmus: Exploring Structure for Meaning, Biblical Theology Bulletin, Volume 17 Issue 2, May 1987, pp. 73-74
4. R. Alan Culpepper, The Pivot of John's Prologue, New Testament Studies, Volume 27, Issue 1, October 1980, p. 16