What is the point of origin for a square area of effect?

Question

The spellcasting rules for areas of effect state:

A spell's description specifies its area of effect, which typically has one of five different shapes: cone, cube, cylinder, line, or sphere. Every area of effect has a point of origin, a location from which the spell's energy erupts. The rules for each shape specify how you position its point of origin. Typically, a point of origin is a point in space, but some spells have an area whose origin is a creature or an object.

A spell's effect expands in straight lines from the point of origin. If no unblocked straight line extends from the point of origin to a location within the area of effect, that location isn't included in the spell's area. To block one of these imaginary lines, an obstruction must provide total cover.

Notably, square is not one of the shapes defined, yet there exist several spells which have a square area of effect, such as entangle or Evard's black tentacles.

The spell grease tells us in its description:

Slick grease covers the ground in a 10-foot square centered on a point within range.

But this clarification is not present in the descriptions of entangle and Evard's black tentacles.

So what is the point of origin of a square area of effect when it is not specified in the spell description?

Answer 1

It's up to the DM.

I see two possible interpretations, based on how to consider a "square":

1. as one of the faces of a cube
2. as a "disk" (under a different distance: see below) centered in the point of origin

The main consequence of adopting one of the above rulings regards the reached area.

Square as a face of a cube

Since a square is the area covered by the bottom face of a cube, one choice consists of applying the rules for the cube area:

You select a cube's point of origin, which lies anywhere on a face of the cubic effect. The cube's size is expressed as the length of each side.

A cube's point of origin is not included in the cube's area of effect, unless you decide otherwise.

One can adapt the above text to square areas:

You select a square's point of origin, which lies anywhere on a side of the squared effect. The square size is expressed as the length of each side.

A square point of origin is not included in the square area of effect, unless you decide otherwise.

A DM can consider a square area as a cube with infinitesimal height, even its effects cover a volume instead of an area.

Square as disk

There is only one spell that specifies that the origin is the center of the square: Grease. This amounts to use a different metric to measure distances on the plane (the distance induced by \$\ell_\infty\$ norm instead of the classic Euclidean norm).

Reached area

The first choice allows to reach space/enemies more far away than the first one. As an example, consider Evard's Black Tentacles: it has a range of 90 ft. and a square area of effect of 20 ft. Adopting the first ruling allows the caster to put the origin at 90 ft and then they can reach points which are at 110 ft. On the other hand, the second ruling allows to put the center at 90 ft but only further 10 ft can be reached, since the remaining 10 ft are below the maximum range.

Which one, then?

The DM should decide, taking into account mainly the issue regarding the reached area and that there is actually just a spell which specifies the origin in the center of the square.

Answer 2

As you've noted, the Spellcasting chapter doesn't mention square area of effect, just cone, cube, cylinder, line, and sphere (which, technically, are "volumes" not "areas" :-).

Given that the point of orgin for a cylinder is the centre of the top or bottom circle and some square spells explictly say the point of origin is the centre, I think it's fair to rule that the point of origin for all square spells is the centre of the square.

Not that there are many of them. Mirage arcane, Evard's black tentacles, entangle, grease?

Answer 3

It is unclear. I would treat it as a cube

Cubes have their points of origin well defined.

You select a cube's point of origin, which lies anywhere on a face of the cubic effect. [...] A cube's point of origin is not included in the cube's area of effect, unless you decide otherwise.

Squares do not.

While Grease specifies the point of origin is the center, the others do not. For my table, the simplest solution is to treat the square as a flattened cube. The point of origin lies on a point on its edge. It feels to my table like a logical ruling, but you should always confirm with your DM and agree as a group on which decision to take.

Answer 4

Ask your DM

Like everything else that isn’t clearly defined in the rules.

The most obvious solutions are:

• treat it like a cube as being the closest shape that is defined.
• use the centre as some spells explicitly do.

Answer 5

It varies by the spell; the DM will make a ruling if needed. Grease specifies the point at the center. Thunderwave, OTOH, is a 15' square with the caster at the center of one side of it. If it isn't specified, as a DM, I'll probably generally let the caster choose whether the point of origin is the center, a side, or a corner.