The Many Systems of D&D

How many systems and mechanics does D&D 5E have? More than you might think! 

Did you know, for example, that time travel is part of the core game in D&D? 

The sphinx in the Monster Manual has a lair action that allows it to move everything within the lair 10 years forward or back in time. Yes, sure, a DM who didn’t want to introduce time travel into their fantasy adventure game could just skip over that particular lair action. But I do think there’s a big difference between choosing not to implement an optional rule (like the Dungeon Master's Guide’s alternate “Time Warp” rule for Feywild Magic) and consciously omitting an effect that’s not labeled optional or variant. Even in a game where DMs can modify or tailor as they see fit – in fact, especially in a game where that’s true – what is and isn’t in the rules, and what the rules do or do not describe as core to the game, is incredibly important. 

The way time travel is casually introduced in the sphinx’s entry got me thinking about how many other mechanics are quietly “back-doored” into the game.

Semantic disclaimer: There’s a lot of squinting and estimating as to what is part of a “system” or what constitutes a “mechanic.” I didn’t count every discrete class feature and monster ability as its own mechanic. Instead, I tried to focus on ludological objects that either affect the game more broadly; or relatively narrow instances that seem to imply unstated or backdoored truths of the game. 

Generally, I think something is probably a system or a mechanic if one or more of the following is true:

1. The players or the DM use it to resolve uncertain situations
2. It formally mediates gameplay in some way that’s recognizable from table to table
3. It’s something that is tracked and that can change over time
4. It triggers or modifies the rolling of dice 

Below is the list I produced after reviewing the three core rulebooks. For most of this list, I merely note the rules object or grouped-together objects. But for a minority I’ve included some thoughts about game design (often implicit, unstated decisions the designers made when creating 5E). I am sure I missed some good candidates. I welcome feedback on my taxonomic approach.

D&D Mechanics

1. Ability Checks, Skills, and Passive Checks
2. Ability Scores and Ability Score Modifiers – D&D mostly uses just the modifiers, but uses the ability scores themselves (for calculating jump distance, certain monsters with drain attacks, etc.) just enough that they can’t be removed from the game entirely. This will be a recurring theme with some other entries in this list.
3. Actions, Reactions, Bonus Actions, Free Interactions, Held Actions
4. Advantage and Disadvantage, Inspiration 
5. Age, Height, Weight – Mostly flavor, but weight can come into play in games where encumbrance is employed and characters want to do something like fireman carry a helpless ally out of danger. Certain spells and magic items also incorporate weight limits.
   Certain monster abilities affect age, like the ghost’s horrifying visage, and another one of the sphinx’s lair actions, but 5E doesn’t have a mechanical effect triggered by growing old like some previous editions did.
6. Alignment – Compared to past editions, most of the mechanical heft has been stripped away; but weirdly, not all of it. You can play many alignment-agnostic sessions in a row before coming across one of the spells, magic items, or monsters that cares about alignment. I can possibly excuse this weird, crufty decision for monsters and magic items, since alignment-adverse DMs can just choose not to put those in their games, or to hack them slightly when they do. But tying alignment to a few spells (e.g. damage type for some cleric spells) seems strange, particularly when something like Protection from Good and Evil has changed to care about creature type instead of, you know, good and evil.
7. Armor Class  
8. Attack Bonus  
9. Attitude
10. Backgrounds
11. Challenge Rating
12. Chases
13. Class, Class Features, Multiclassing, and Archetypes (Paths, Colleges, Domains, etc.)
14. Coins and Wealth   
15. Conditional Immunities   
16. Conditions and Exhaustion – Note that certain effects that feel like conditions do not inflict a condition as defined in the rules. Confusion is a good example. The spell Confusion randomizes behavior on a 1d10. The faerie dragon’s euphoria breath ability works similarly, but with a 1d6 rolled to reflect only two choices. I guess this change reflects that this friendly, prank-oriented creature doesn’t want to make people stab each other.
    The gibbering mouther and the umber hulk have a bespoke 1d8 table different from both the confusion spell and euphoria breath; the odds are different from the spell Confusion, and the 20% chance to act normally for the turn is removed. This minor tweak hardly seems worth carving out separate mechanical space. 
17. Cover and Hitting Cover   
18. Creature Type, Tags, Races, Subraces, Demon/Devil Types, Templates, Variants – A wild hodgepodge of classifications for creatures. Note the incredibly tiny ooze creature type, which in the original books only includes the core four wet boys grouped together under the ooze entry in the Monster Manual. Why weren’t they just treated as aberrations or monstrosities?
19. Critical Hits and Failures 
20. Curses – Like Confusion, “cursed” is kind of a shadow condition, providing an overlay for other effects, similar to how charmed often does. Reference monster abilities like the formorian’s Curse of the Evil Eye, the Lamia’s Intoxicating Touch, and the lycanthrope's… lycanthropy. See also the few cursed magical items (e.g., Berserker Axe) that have survived into D&D 5E. These effects don’t seem to have a particular mechanical link except that they can be treated with Remove Curse.
21. Damage, Damage Types, Damage Resistances, Damage Immunities 
22. Death, Massive Damage, Death Saves, Instant Death   
23. Difficulty Class 
24. Disease and Poison  
25. Downtime (Carousing, Crafting, Strongholds)
26. Encumbrance and Weight  
27. Environment, Terrain, Planes of Existence    
28. Equipment  
29. Feats and Boons  
30. Flanking, Diagonals, Facing   
31. Hit Dice and Healing Surges  
32. Hit Points, Temporary Hit Points   
33. Honor and Sanity; Madness, Fear, and Horror; Renown
34. Ideals, Bonds, Flaws, Traits… and Secrets? – Page 90-91 of the DMG gives villains traits, ideals, bonds, and flaws, much like players… but that last one is Flaw or Secret. Secrets are also mentioned in the Nothic’s stat block, implying that all characters (may? should?) have a secret.   
35. Initiative and Surprise  
36. Languages   
37. Legendary Resistance, Lair Actions, Legendary Actions, Regional Effects  
38. Lingering Injuries – The troll’s Loathsome Limbs variant rule could be flagged here as a little backdoor hit location system.     
39. Loyalty   
40. Magic Items and Attunement  
41. Morale  
42. Nature (Elemental, Undead, Constructed, Immortal, etc.) – These are placed outside the statblock in the Monster Manual, with the flavor/lore text, but they also have clear mechanical implications (e.g. a creature that doesn’t need to breathe couldn’t be suffocated with the Control Water spell or similar immersion). They also suggest some subtle differences between similar creatures; for example the Galeb Duhr and Gargoyle are elementals on successive pages of the Monster Manual, but the latter has Elemental Nature while the former does not, even though the Galeb Duhr can “remain perfectly still for years a a time.” Presumably the Galeb Duhr just has a slow metabolism. Ooze Nature merely allows oozes to skip sleep. I had assumed they eat -- they are predators/scavengers, after all – but do they breathe as well? Will-o’-wisps don’t require air, hydration, or sleep, but by exclusion presumably do require “food” (e.g. their Consume Life power); but other incorporeal undead like the Wraith and the Shadow, with their drain abilities, don’t require “food.”    
43. Optional Combat Maneuvers (climb onto a bigger creature, disarm, mark, overrun, shove aside, tumble, cleaving through creatures)   
44. Pantheons and Domains   
45. Plot Points  
46. Proficiency (Armor, Tool, Weapon, Skill, Saving Throw)   
47. Proficiency Bonus, Expertise, Proficiency Dice, Hero Points, Background Proficiency, Personality Trait Proficiency – Note how proficiency bonus, level, and tier all layer different degrees of specificity / granularity onto the advancement process. 
48. Random Encounters   
49. Range  
50. Rests, Limited Usage X/Day, Recharge X-Y, Recharge after a rest  
51. Saving Throws   
52. Sense, Darkvision, Vision and Light   
53. Size  
54. Speed, Movement, Special types of movement, Underwater Combat   
55. Spellcasting, Spells, Spell Level, Spells Known, Spell Slots, Spell Points, Components, Psionics, Ritual Casting    
56. Souls, Memories – Souls occasionally surface as character features that can be attacked (Trap the Soul, the Night Hag’s Soul Bag) and memories crop up in various places (Modify Memory, Mind Blank, the Gas Spore’s Beholder Memories, Speak with Dead) 
57. Spellcasting Ability, Spell Save DC, Spell Attack Bonus    
58. Success at a Cost and Degrees of Failure – These are the framework for whole   mechanics in other roleplaying games, where these the two principles are at the crux – rather than the edges – of the action. D&D does occasionally uses them within the cor rules, usually as a “fail by 5 or more” stipulation on effects like the Medusa’s petrification and drow sleep poison.       
59. Suffocation – Like confusion and cursed, this is another backdoor condition not formally identified as such
60. Time, Rounds, Days, Time Travel 
61. Time Travel (Sphinx lair action, Fey Time Warp)
62. Weapon Durability / Degradation – just as the Troll’s Loathsome Limbs backdoor hit locations into the game, the Rust Monster’s signature ability introduces a form of weapon degradation.
63. XP and Levels