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We're using a grid-based combat system for combat. The party cleric is learning the joys of Spirit Guardians, which has a range of Self (15-foot radius):

You call forth spirits to protect you. They flit around you to a distance of 15 feet for the duration.

Again, as we're using a grid, the cleric picks one of the four intersections around her to center the spell effect off of, per the guidance on adjudicating areas of effect on a grid on DMG p. 251:

The area of effect of a spell, monster ability, or other feature must be translated onto squares or hexes to determine which potential targets are in the area and which aren't. Choose an intersection of squares or hexes as the point of origin of an area of effect, then follow its rules as normal.

Which intersection she chooses has an effect on the enemies who might be caught within the spell.

15-foot-radius area of effect for spirit guardians on a grid, showing the cleric (Faith) in the 5-foot square northeast of the spell's point of origin, and a hell hound in the square to the east of the cleric

The spell moves with the caster's "self" during the duration/concentration. Two questions:

  1. As the caster moves, can the relative intersection they chose be changed? For instance, if the cleric chose the lower right intersection to their square to be the center of the radius when cast (as in the image), and then moves three squares to the left, does the Spirit Guardians "circle" remain centered on that lower right intersection? Or can they change that to be (e.g.) the upper left intersection?

  2. If the answer to #1 is "Yes":
    Can the cleric change the placement of the spell's area without moving (thought of, perhaps, as leaning to one corner of their 5-foot square vs. another corner)?

Note: There are aspects of this question that may be already partially answered, depending on which of the answers one selects here: Does the radius of the Spirit Guardians spell depend on the size of the caster? (and in connected questions)

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The DM needs to decide how this works

As it is with anything that comes out of the Dungeon Master's Guide, rules about D&D on a grid comes down to the DM's discretion, and the 'rules' in the DMG are nothing more than suggestions/guidelines for newer DMs. They don't cover all corner cases, and probably would be bad if they tried to.

I have experience, as a DM and as a player, playing on a grid, so I'll give some advice on how I would adjudicate these questions.

First, the two questions in your list:

  1. Generally, I wouldn't allow a player to "change corners" on the grid relative to their token. My group uses a Virtual Tabletop, and if the player were to ask "can I use the upper-left corner instead of the lower-left corner?", I'd generally allow it; but I wouldn't be allowing them to shift it freely during the duration of the spell. If they choose the lower-left corner as the "center" of their Spirit Guardians effect, they need to keep that location unless and until the spell is cast again, even if the character moves around
    • The obvious corollary, of course, is something where a spell effect is intrinsically tied to the character's location, but is still pinned to an object, like the Light spell (PHB 255), which might be attached to an object that's on a character's person. If they told me "I take the handbook and hold it out as far in front of me as I can, to try to extend the light," I'd probably have no problem nudging the light source to a different corner of the character's cell to represent that, although depending on the context I might insist that this requires the use of the character's action.
  2. So since I generally wouldn't allow this for moving characters, it logically follows I wouldn't allow this for stationary characters either.

You don't need to use corners all the time, though

I actually don't agree with the DMG advice to always choose a corner of the cell to center effects. That isn't to say you should never use a corner, and I'm guessing the reason they give that advice is because it makes it easiest to figure out things like the radius of a spherical spell effect; see the example screenshot here, where a "5' radius effect" has a very different shape depending on how the effect is centered:

Screenshot, from FoundryVTT, showing that a 5-foot-radius spell effect centered on a vertex cleanly targets the surrounding four cells; but a 5-foot-radius spell effect centered within a cell instead targets a plus-shape; and putting it on the edge of a cell targets only 2 cells! The VTT I use decides that a cell is "inside" the AOE if the circle includes the center of the cell, and "outside" the AOE if the circle does not include the center of the cell.

It's clear from this example that putting the spell effect's center on a corner just "looks" the cleanest, and probably is the easiest for DMs and Players to understand. But one thing that's also clear is that this difference matters a lot less when you're dealing with larger AoEs on spells.

Screenshot, from FoundryVTT, showing how once you're dealing with 10-foot-radius effects, the differences between targeting corners vs edges vs centers becomes much less severe

Even as small as 10' radius spells, we see that the difference is a lot less pronounced, and once you're dealing with stuff like the ranges of Fireball (20' radius) or other larger spells, the choice of whether to put the AOE on a center, edge, or corner is much less meaningful.

Of course, if you place a premium on "tidiness", the corner probably still wins out, and I'd respect any table that prefers to just use corners. My only point here is that once you're dealing with larger AoE effects, like those of Spirit Guardians and its 15' radius, the choice of whether to use corners or centers becomes less important. So you have the option of just using centers instead, and reducing the need for players to try to game the corner positioning.

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    \$\begingroup\$ You may want to specify how you are determining which squares are purple. The DMG uses the 50+% covered rule, I'm not sure what you're using though \$\endgroup\$ Oct 5 at 20:59
  • \$\begingroup\$ @Medix2 Good point. I'm using whatever I configured months ago in Foundry; I'll double check and see what it's doing. It looks like the rule is "is the center of the cell covered by the circle?" as the check. \$\endgroup\$
    – Xirema
    Oct 5 at 21:01
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    \$\begingroup\$ @Medix2 It looks like this method is somewhere between the DMG “at least half rule” and the XGtE “any part of the square” rule for templates, which reads: “If any part of a square is under the template, that square is included in the area of effect.” So while this answer doesn’t utilize either of the contradicting suggestions in the official optional rules, it is based on experience and appears consistent, which makes this a good answer. \$\endgroup\$ Oct 5 at 21:14
  • \$\begingroup\$ @ThomasMarkov Oh yeah, agreed, I already upvoted it, I was moreso just curious \$\endgroup\$ Oct 5 at 21:24
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    \$\begingroup\$ "They don't cover all corner cases" I know this isn't intended as a pun, but it made me giggle quite a bit. \$\endgroup\$
    – T. Sar
    Oct 5 at 22:37
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"They flit around you to a distance of 15 feet for the duration." That's 15 feet in every direction not from just one corner of your square. When an AOE spell has a target of self you're not choosing one single point on your square, the effect is centered on the entirety of the square you occupy or multiple squares in the case of larger creatures. So for Spirt Guardians the effect would cover a 7x7 square for medium creatures or an 8x8 square for large. And they should be squares. In 5e there is no reduction for diagonal movement, a move diagonally on the grid is the same 5 feet of movement as moving straight. In 5e circles are square...

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  • \$\begingroup\$ Welcome, please add the source of your reasoning to your answer - such as a citation of the rule or a reference to its page numbers. :) \$\endgroup\$
    – Akixkisu
    Oct 7 at 6:47
  • \$\begingroup\$ The question is asking how to use one particular rule: "Choose an intersection of squares or hexes as the point of origin of an area of effect", are you saying "just don't use that rule"? \$\endgroup\$ Oct 7 at 12:34
  • \$\begingroup\$ The question specifically mentions the spell Spirt Guardians. I'm saying you don't use that rule with this spell. Spells that target a single point will say so in the spell description. The Fireball spell for example- "A bright streak flashes from your pointing finger to a point you choose within range..." Whereas the Spirt Guardians spell states- "You call forth spirits to protect you. They flit around you to a distance of 15 feet for the duration." Not 15 feet from a point, 15 feet around you. In the first example pictured above the AOE is clearly not 15 feet around the spell caster. \$\endgroup\$ Oct 8 at 22:42

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