The Book of Mormon is a composite work, compiled from several archaic records that were abridged ultimately by Mormon and his son Moroni. . . . According to the analytical work of Roger Keller, Mormon and Moroni each display their own unique and distinctive editorial styles. When Mormon is acting as an abridger, he interacts extensively with the underlying documents he is abridging. It is not always immediately possible to distinguish Mormon's own words and comments from the words that he draws from the materials he is condensing. As one reads along in many sections abridged by Mormon, one often senses that a subtle shift has taken place as a smooth, almost imperceptible transition has occurred from the underlying historical narrative to Mormon's commentary on that narrative. By carefully backtracking, one can discern, however, where the transition was made.
Moroni, on the other hand, interacts far less extensively with the text he is abridging. Moroni is usually careful about marking the beginning and ending of the comments that he has inserted into the abridged record. For example, his comments in Ether 3:17-20, 4:1-6, 8:18-26, and 12:6-13:1 are readily distinguishable from the abridged portions in the book of Ether. His frequent use of the phrase "I, Moroni" in Ether 1;1, 3:17, 5:1, 6:1, 8:20, 8:26, 9:1, 12:6, 12:29, 12:38, and 13:1 makes it easy to tell what Moroni has written and what he has abridged. [Roger P. Keller, "Mormon and Moroni As Authors and Abridgers" in Reexploring the Book of Mormon, pp. 269-270]