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Playing on a grid, imagine a 10-foot-wide hallway. Can a Medium creature stand between the grid squares, effectively leaving 2.5 feet on each side? Or does the creature have to "hug the wall" and leave a full 5 feet on one side?

Here is the relevant basic rules section on "Movement and Position".

I'm asking this for two reasons:

  • mundane "block the corridor by standing in the middle" situation (would seem to block it for Medium creatures, and force Small creatures to squeeze, i.e. use extra movement)
  • blocking the corridor from a frightened creature already standing next to you (if there's a 5-foot-wide gap, they can circle you, but squeezing through a 2.5-foot-wide gap next to the very creature they fear seems like a stretch...)

A Medium creature can block the middle 5-foot-space of a 15-foot-wide hallway, preventing Large creatures from passing, if I understand the rules correctly. Medium creatures not being able to block middle 5 feet of a 10-foot-wide hallway seems arbitrary, or an artefact of the grid system. In theater-of-the-mind combat, things would probably work consistently (either blocking the hallway in both cases or not blocking it in either case). The hallway changing its tactical significance between these modes of battle seems contradictory.

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  • \$\begingroup\$ @WakiNadiVellir the "must be careful because within sword reach" is the reason you can't pass a Medium creature in a 10ft hall without provoking an opportunity attack. \$\endgroup\$ – Erik Nov 2 '20 at 14:36
  • \$\begingroup\$ @Erik Passing a creature does not provoke an opportunity attack. Leaving their melee reach does. \$\endgroup\$ – WakiNadiVellir Nov 2 '20 at 15:32
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Don't get tangled in the particular abstractions

Your intention is to block a corridor, about three metres wide, by standing in the middle (and presumable striking or tripping or blocking opponents as they come?). Clearly this is a reasonable thing to try. Now, the group, probably with the game master in charge, has to figure out how to represent this within rules framework in use.

Maybe you are preparing to attack or push anyone trying to pass you (ready an action)? This work regardless of the particulars of the grid in use. Just do it.

Is it credible that just by standing over there there is not enough space for others to get past? Three meters is quite wide, so unless you have an almost three meter wide barrier with you, quite unlikely. But three meters is narrow enough that enemies can't get past you without worrying, especially if you are in the middle. The game represents this in a somewhat strange way by opportunity attacks. Use them (or house rule if they do not satisfy your group).

If, for some reason, it is vitally important that you are in fact standing right in the middle of the corridor, and it is vitally important to use a grid here and now, consider the following alternatives:

  1. Align the grid so that one grid is in the middle. The corridor can be diagonal.

  2. Align the grid so that one grid is in the middle. The corridor can consist of half-squares.

  3. Use a hexagonal grid instead and place it so that a hex is in the desired position in the middle of the corridor.

In any of these cases, you might want to have to adjudicate things such as partial squares or hexes.

So it might be easier to ignore the grid for a while or to just assume that there is a small local change in the grid which allows someone to stand in the middle, but still so that only two people can fight side by side in the corridor (when someone comes to one side, their attacks force you to take a step to the other, so that both are located in a definite square). And if an area of effect would hit precisely one of those squares and you are in the liminal stage between them, roll d2 to see if you are affected or roll your saving throw with advantage or take only half damage, whichever suits the situation best.

Summarizing: Use the rules to model the current fictional situation in a satisfactory way, rather than taking the abstraction the rules cover as a ground truth.

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You can't block a 10-foot-wide hallway, regardless of the grid

Rules as written, a Medium creature controls a 5-foot-by-5-foot space. When you play on a grid but try to "stand between squares", you're effectively trying to control a 10-foot-by-10-foot space, which you'd need to be a Large creature to do.

Playing on a grid is an optional rule. If you use it, you move square by square (PHB, p. 192):

Rather than moving foot by foot, move square by square on the grid.

Narratively you can stand in the middle of a 10-foot-wide hallway, but when you end your turn, you have to put the miniature in a free square. You don't "hug the wall" this way. Instead, you control a 5-foot-by-5-foot space. See the "Space" subsection of the rules (PHB, p. 192):

A creature's space is the area in feet that it effectively controls in combat, not an expression of its physical dimensions. A typical Medium creature isn't 5 feet wide, for example, but it does control a space that wide.

Even if you don't play on a grid, you still can't block a 10-foot-wide hallway, unless your size is Large, since a Small or Medium creature controls only a 5-foot, not 10-foot, space. The "Playing on a Grid" variant rule is consistent with this restriction.

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    \$\begingroup\$ I think it is not against RAW to have a 7½' hallway. So how would that work then? Or is it actually impossible, mechanically a hallway must be classified as either 5' or 10' regardless of the "actual" narrated/drawn width? \$\endgroup\$ – WakiNadiVellir Oct 31 '20 at 11:02
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    \$\begingroup\$ 7½' hallway is perfectly normal. But if you play on the grid, you have to align. Maps are usually optimized for the grid, if the grid is assumed. \$\endgroup\$ – enkryptor Oct 31 '20 at 11:16
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    \$\begingroup\$ The reason I find this answer unconvincing is the first paragraph. According to it, rules forbid a medium creature from "controlling" a 10' hallway against a medium creature. Yet a large creature could "control" 20' hallway against another large creature, without any special rules about it. Same for a medium creature in the middle of a 15' hallway against a large creature. So clearly controlling an area larger than your "grid size" is possible. \$\endgroup\$ – WakiNadiVellir Oct 31 '20 at 16:29
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    \$\begingroup\$ The rules are not a simulation of reality, they are game rules to make the game fun to play. Sometimes they don't entirely make sense to you, and you can play along or change the rules (which I wouldn't do for a situation that really comes up, but up to you and your table) \$\endgroup\$ – Martin Epsz Oct 31 '20 at 18:11
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    \$\begingroup\$ @WakiNadiVellir I see no contradiction among the situations you describe. The creatures you describe are controlling (in the rules sense) their specified grid sizes. How that impedes particular opponents in particular situations has to do with the parameters of those opponents and situations, and might cause phenomena that look like controlling space (in an informal sense); but it doesn't change what space the creature controls (in the rules sense). \$\endgroup\$ – Greg Martin Oct 31 '20 at 21:46
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A Medium creature can guard a 10-foot-wide passage, if they can stand in the middle.

A Medium creature controls a 5ft area. Another Medium creature which they are hostile to, cannot pass through this space using its movement. (A friendly creature can pass through this space, but cannot end its turn there.)

  • A creature can squeeze through a space that is large enough for a creature one size smaller than it. Thus, a Large creature can squeeze through a passage that's only 5 feet wide.

So a Medium creature can squeeze through a 2.5ft gap, & a Large creature can squeeze through a 5-foot gap... however:

  • While squeezing through a space, a creature must spend 1 extra foot for every foot it moves there, and it has disadvantage on attack rolls and Dexterity saving throws. Attack rolls against the creature have advantage while it's in the smaller space.

Now, consider that almost any Medium creature has a 5-foot reach, in addition to the space it controls. Any hostile creature within 5 feet of that creature's space provokes an opportunity attack if it leaves that 5-foot reach.

If a Medium creature with a 5-foot reach stands in the middle of a 15-foot-wide passage, it is able to use its reaction to make an opportunity attack against any hostile creature that passes by.

If a Medium creature with a 5-foot reach stands in the middle of a 10-foot-wide passage, it it is able to use its reaction to make an opportunity attack against any hostile creature that passes by, and a Medium creature passing by is "squeezing through", & therefore spends 5 extra feet of movement, & has disadvantage on Dex saves & any attacker gets advantage against them; so the guard gets a good chance to attack any Medium-sized enemy. (If the guard has Readied the Grapple action, they can attempt to halt the enemy's movement instead.)

If a Medium creature with a 5-foot reach stands in the middle of a 15-foot-wide passage, a Large creature can squeeze through the 5 feet on either side; spending an extra 5 feet of movement, & provoking an opportunity attack with advantage.

If a Medium creature with a 5-foot reach stands in the middle of a 10-foot-wide passage, a Large creature cannot squeeze through the 2.5 feet on either side. (The Large creature might use its action to shove the Medium creature though, & then use remaining movement to pass by.)

The 5-foot grid is an optional rule, but even on a grid this can still be possible:

  1. The player & GM decide precise positioning, not the grid. If a player specifies that they are pressed against a wall, or in the middle of a space, it is the GM's discretion to dictate what is within reach. The GM can put things directly on grid-lines if they want, & likewise can allow a player to do the same; especially if the location is easily understood, like "the middle of a hallway"!

  2. A 10-foot wide hallway could be positioned so that 5-foot spaces run the middle of it, with the spaces on either side cut off at 2.5 feet each.

Either way, a Medium creature only "blocks" a 5-foot space, but any other creature passing within 5 feet of that space, on either side, might get killed.
(Or if the Medium guard has Grapple readied, the other creature may see its movement become 0.)

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    \$\begingroup\$ "So a Medium creature can squeeze through a 2.5ft gap" technically one size smaller than Medium is a Small creature which still takes a 5ft space. A Small creature can squeeze through a 2.5ft gap but I don't think a Medium creature would be able to by these rules. (See: Size categories table on page 191 of the PHB) \$\endgroup\$ – smbailey Nov 2 '20 at 20:07
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    \$\begingroup\$ block is not the same as make an attack of opportunity on, A creature can just eat the attack of opportunity am move through the space, or use an ability than denies AoO, and once you have made your one AoO other creatures don't even have to worry about that. Also you can't grapple as an AoO, rpg.stackexchange.com/questions/57368/…. \$\endgroup\$ – John Nov 3 '20 at 17:14
  • \$\begingroup\$ Thanks for pointing that out! Quite correct: Grapple does NOT equal an Attack roll; it's an Ability roll. The intruder gets Disadvantage on Dex saves if Squeezing Through, but the guard has to be a turn ahead, to Grapple rather than do AoO. And (barring something awesome) AoO is just one Attack, after all... I've survived worse! \$\endgroup\$ – ProphetZarquon Jan 12 at 21:26
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In terms of grid position, no.

You're either in the left square or in the right square, and you only control the square you're in. If you specify that you stand in the exact middle of the hallway, the DM is supposed to ask you which side you're controlling, and place your mini accordingly.

The grid is an abstraction. There shouldn't be anything stopping you from standing in the exact middle of the hallway and controlling that space. But controlled space is also an abstraction. You don't have a 5-foot-diameter rigid force field preventing anything from entering; you have an area that you can easily defend, and it's about 5 feet across. And the hallway is 10 feet wide, so if someone wants to get past you, they can. (You do get an opportunity attack, but they can negate that just by keeping their guard up as they walk through.)

That's really what the grid is telling you: the opening is 2 grid units wide, so it takes 2 medium creatures to block it. Where exactly they're standing is somewhat fluid, but either you have enough guys to control the space or you don't.

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    \$\begingroup\$ Imagine the same situation scaled up. So a Large creature is guarding a 20' hallway against another large creature. In this case the creature can stand directly in the center on a regular grid, and it does block the opponent from passing by. So why would it be different for a Medium creature standing in the center of a 10' hallway? (i.e. only having 2.5' of space on each side does not allow the Medium size opponent to pass by.) \$\endgroup\$ – smbailey Nov 3 '20 at 17:27
  • \$\begingroup\$ @smbailey It's different because the grid rules are built around Medium creatures. Yeah, I know. \$\endgroup\$ – Mark Wells Nov 3 '20 at 17:58

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