How does a DM know, in D&D 5th edition, which source to refer to if there is a seeming conflict between two core rule books?

During the discussion under this question, I noted the following assertion made that seems right, but I'd like to see a formal derivation of the position.

MM supersedes PHB for stats as well as what Miniman states, PHB doesn't offer "Variant" which is the DMs choice. — this comment by XAQT78


The MM supersedes the PHB. — this answer by Ryan

While my own instinct is that the Monster Manual is a DM tool, and DM's rulings overcome other opinions and interpretations, thus MM > PHB, new DM's may be at a loss when running into a similar case where one core book leads them in one direction, and another core book leads them in a different direction.

An early discussion on Inspiration, where the tone in the PHB differs somewhat from the tone in the DMG, is a similar case.

If the MM supersedes the PHB, please explain the reasoning behind why. (Or the reverse).

  • Example: The example I am most familiar with, as I have the original printings, was the discrepancy between the MM and PHB stats for bears Appendix D: Creature Statistics(PHB 1st printing) where the bonus was +3 and +5 respectively. It has since been modified by PHB errata of 2018 to match the MM,

    [New] Black Bear (p. 304). The to-hit bonus for both Bite and Claws is now +4. [New] Brown Bear (p. 304). The to-hit bonus for both Bite and Claws is now +6. (errata

    but for just under 4 years the question could have been: which has precedence?

Note: this question does not intend to address things like the Homunculus, that in the MM does not have a discrete means of creation, while the expansion/supplement Xanathar's Guide to Everything adds such a spell/process for the PC's to use. The question's scope is limited to the core books: Player's Handbook, Monster Manual, Dungeon Masters Guide.

If a precedent was set in previous editions ...

... and this point of view is a carry over from 3.xe, and/or 4e, that would be useful in an answer.

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    \$\begingroup\$ A flat-out and crystal-clear example of sources actually conflicting may make the question stronger. \$\endgroup\$ – Hey I Can Chan Jan 28 '19 at 13:23
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    \$\begingroup\$ The examples are from other questions' comments and answers. I think a question works better if it stands alone; that way the reader doesn't have to sort through other questions to find examples of why this question is important. (Also, we 3.5e folks know this problem well; I feel your pain.) \$\endgroup\$ – Hey I Can Chan Jan 28 '19 at 13:29
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    \$\begingroup\$ I think you're onto something here, but without a specific issue to compare the books against this feels too broad. I definitely see the case as to why it may not be too broad, but without a concrete example to review I'm not sure how to address this. \$\endgroup\$ – NautArch Jan 28 '19 at 14:13
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    \$\begingroup\$ @PJRZ The PHB contains several monster statblocks that could well be in conflict with the MM ones. There is an overlap in their domain, even if slight. Other candidates include spell names and levels. \$\endgroup\$ – kviiri Jan 28 '19 at 14:17
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    \$\begingroup\$ @HeyICanChan To my knowledge they have not, which in my mind makes it very important that this question is based on an actual problem that exists now and not a hypothetical one. If we are going to get into the weeds of source primacy, we need to know there is a reason to do so. "Futureproofing" should not be used as a reason to answer a question without a demonstrable problem at its core. \$\endgroup\$ – Rubiksmoose Jan 28 '19 at 15:07