In SumBible’s model the Doctrine and Covenants is treated as a single book — “Sections” — and this summary concerns that book’s structure. (The doctrinal substance of the D&C is taken up in the canon’s summary.)
The defining structural fact is its chronological arrangement. The 138 sections are ordered, with few exceptions, by the date they were received. Most respond to a specific situation — a question raised, a decision that needed direction, a conflict that needed resolving. Read in sequence, they let a reader watch the Restoration’s organization and doctrine take shape almost in real time. Each section is preceded by a brief editorial heading that records its date and circumstances — a study aid supplied by the Church’s editors, not part of the revelation itself.
A few sections serve as structural landmarks. Section 1 is the Lord’s own preface, placed first though not the first received (Sections 1↗). Section 76 records the vision of the three degrees of glory (Sections 76↗). Sections 84 and 107 lay out the priesthood and its offices; Sections 124 through 132, received in Nauvoo, concern the temple. Section 135 is unusual — a historical record of the martyrdom of Joseph and Hyrum Smith, written by John Taylor rather than received by Joseph Smith.
Set apart from the numbered sections are the two Official Declarations. Official Declaration 1 (1890) ended the practice of plural marriage; Official Declaration 2 (1978) extended priesthood ordination to all worthy male members.
Because so many sections are tied to particular events, they reward being read alongside their history. The Joseph Smith Papers Project and the Church’s Revelations in Context series are the standard study companions.