The unsealed part of the plates contained two major sets of records: the plates of Mormon and the small plates of Nephi. Anciently, Moroni’s father, Mormon, abridged a detailed chronicle of his people—the large plates of Nephi—into a smaller record: the plates of Mormon. He then bound in with them the small plates of Nephi, an unabridged record that covered part of the same time period but focused on spiritual matters.
Before Mormon died in battle, he gave this combined set of plates to Moroni, who engraved on them an abridged account of the Jaredites, the people who had preceded the Nephites in the Western Hemisphere but had become extinct. Moroni added his own history and reflections before burying the plates where Joseph found them.
The sealed portion of the plates came from a record made by the first Jaredite prophet, who was known as “the brother of Jared.” Because Moroni abridged this record, he knew its contents and referred to them in his writings. Other prophets did the same.