The final part of The Spirit of the Laws is dedicated to two studies in the history of law. Book 27 is entitled the “origin and revolutions of the Roman laws on inheritance,” while Books 28-31 (except Book 29) extensively discuss Frankish feudal laws and the creation of civil laws amongst the French. In Book 27, Montesquieu’s discussion of Roman inheritance includes a historical account of various missteps and corrections in inheritance law, the right of testaments, trusts, etc. Book 28 includes a genealogical account of “the origin and revolutions of the civil laws among the French” (532). He compares and contrasts the laws of various Germanic peoples (like the Merovingians, the Carolingians, the Visigoths, and so forth). These Germanic laws were in legal dialogue with Roman law after the fall of the Roman empire. Montesquieu extensively examines the minutiae of laws regarding trial by combat. These books do not serve to further articulate the thematic core of The Spirit of the Laws but should be viewed, instead, as case studies in legal history and comparative law.

Book 29 does not fit this mold. It stands out in Part 6 as a crowning moment of Montesquieu’s treatise, at least insofar as it provides a series of conclusions “on the way to compose the law” (the title of Book 29).