It's a common observation, beloved by Reformed folk, that Deuteronomy reflects ancient near eastern suzerainty treaties--international covenants enacted between a superior suzerain power and inferior vassal state. Such treaties follow what we now think of as standard covenant formulary: a preamble introducing the parties and historical prologue tracing the relationship between them, followed by an outline of the terms, an enumeration of blessings and curses, and miscellaneous other items like calling on witnesses, instituting some sort of public sign, and making provisions for depositing an official copy of the treaty and holding a renewal ceremony from time to time.
Deuteronomy fits the pattern extremely well, making it, in W. L. Moran's words, "the biblical document of the covenant par excellence." A rough outline looks like this:
Preamble and prologue: Dt 1:1-4:43
Terms: Dt 4:44-26:19
Blessings and curses: Dt 27:1-28:68
Miscellaneous other items: Dt 29:1-34:12
This is conventional wisdom, probably familiar to most of you. One bit that remains unsettled, however, is how to outline that long middle section that presents the terms or stipulations of the covenant--the body of the book.
One popular approach is to divide this section between general stipulations and specific stipulations at Dt. 12:1. There are good reasons to do so but another approach, perhaps compatible with he former, is even more helpful. Advanced by Stephen A. Kaufman (The Structure of the Deuteronomic Law, 1979) and others after him, including my fellow RTS faculty member John Currid (Deuteronomy in EVP, 2006), this alternative approach views Dt 4:44-5:33 as a summary statement of the moral law set out under ten heads (the decalogue), and Dt 6:1-26:19 as an explication and application of each head of the moral law to Israel. In other words, Dt. 6:1-26:19 is a kind of divine commentary and practical guide to the decalogue. This highlights the continuing priority of the ten commandments as a summary of the moral law within this administration of the covenant of grace--a priority, originating in Eden, that carries over to the new administration of that same covenant of grace, as Rick Phillips recently noted here.
Here is the most common way of outlining this part of Deuteronomy on this view:
First: Dt 6:1-11:32
Second: Dt 12:1-32
Third: Dt 13:1-14:21
Fourth: Dt 14:22-16:17
Fifth: Dt 16:18-18:22
Sixth: Dt 19:1-22:12
Seventh: Dt 22:13-23:14
Eighth: Dt 23:15-24:7
Ninth: Dt 24:8-16
Tenth: Dt 24:17-26:19
To read Deuteronomy this way is insightful. Consider the light it sheds on the relation of the first commandment to election (Dt 7:1-26), or the third commandment to those peculiar dietary laws (Dt 14:1-21). It's also instructive to read the laws related to tithes in Deuteronomy 14:22-29 as an application of the command to rest in God or the regulations concerning judges, kings, priests, and prophets (Dt 16:16-18:22) as applications of the fifth commandment and those concerning cities of refuge, war, and tasseled garments (Dt. 19:1-21:23) of the sixth commandment.
The pattern fits very well until we reach the transition from the eighth to the tenth commandments. The issue lies with identifying what portion of Deuteronomy 23:15-26:19 deals with the ninth commandment.
Deuteronomy 24:8-16, the oft-proposed candidate, is not very satisfying. Though Currid follows this division, his comment on Deuteronomy 24:8-9 on skin diseases is telling: "It is uncertain how this law fits into an exposition of the Ninth Commandment." Though the remaining six verses on collateral and paying day-laborers has a little better claim to taking up ninth commandment issues, I think the primary concern of this set of "miscellaneous laws" remains on "justice in contracts and commerce, . . . rendering to everyone his due," which is how the Westminster Larger Catechism Q&A 141 summarizes the scope of the eighth commandment.
This concern, it seems to me, continues through Deuteronomy 25:12. It's not until Deuteronomy 25:13-16, concerning the use of just weights in the marketplace, that we encounter a passage that seems to be a solid candidate for the ninth commandment's concern for honesty or "the preserving and promoting of truth between" people (WLC Q&A 144). But even this passage on fair weights and measures falls under the eighth commandment's focus on justice in commerce.
It's not surprising to find one kind of activity covered by more than one of the ten commandments. Using such an activity as a transition from one to the next also makes sense--Deuteronomy 13:1-18 is another example. But this observation suggests another possible solution to the riddle of the ninth commandment in Deuteronomy: What if the ninth commandment receives no distinct discussion but is a moral enthymeme of sorts? Perhaps the point is that truth inwardly and outwardly is a necessary condition of keeping all the other commandments. If so, then the treatment of the ninth commandment in Deuteronomy 6:1-26:19 may be distributed over the elaboration of the first eight commandments. If you've been studying your way through those, by the time you reach the ninth commandment its elaboration is obvious and requires no distinct discussion.
Whatever we make of the riddle of the ninth commandment in Deuteronomy 6:1-26:19, it is clear that honesty or truth inwardly and outwardly is a necessary condition to keeping the rest of the commands--to being the kind of person we were created to be and have been redeemed to be by the one who is "full of . . . truth" and is "the truth" (Jn 1:14; 14:6).