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In follow up to my question How can I get greater coverage using Alarm and can it be cast more than once?, I would like to find out whether this portion implies shapability and what restrictions are placed on shapeability. And can the same logic be applied to a possibly game breaking point?

Choose a door, a window, or an area within range that is no larger than a 20 foot cube

It seems that this language implies shapability because the wording is "an area". But does no larger than a 20 foot cube mean no measurement can be larger than 20 feet? Or does it mean the area or volume cannot exceed that of a 20 foot cube? We already know the size can be round or rectangular (windows can be either).

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Basic Area of Effect Rules

As far as Area of Effect goes...

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.

The linked section goes on to explain exactly what each of the five basic shapes means, and none of them allow for shaping.

Shaping

One of the design precepts of 5th Edition is plain language - in other words, spells, features, and traits only do what they say they do. Take these spell descriptions for example...

  • Wall of Fire

    You create a wall of fire on a solid surface within range. You can make the wall up to 60 feet long, 20 feet high, and 1 foot thick, or a ringed wall up to 20 feet in diameter, 20 feet high, and 1 foot thick.

  • Wall of Force

    An invisible wall of force springs into existence at a point you choose within range. The wall appears in any orientation you choose, as a horizontal or vertical barrier or at an angle. It can be free floating or resting on a solid surface. You can form it into a hemispherical dome or a sphere with a radius of up to 10 feet, or you can shape a flat surface made up of ten 10-foot-by-10-foot panels. Each panel must be contiguous with another panel. In any form, the wall is 1/4 inch thick.

  • Wall of Ice

    You create a wall of ice on a solid surface within range. You can form it into a hemispherical dome or a sphere with a radius of up to 10 feet, or you can shape a flat surface made up of ten 10-foot-square panels. Each panel must be contiguous with another panel.

  • Wall of Light

    A shimmering wall of bright light appears at a point you choose within range. The wall appears in any orientation you choose: horizontally, vertically, or diagonally. It can be free floating, or it can rest on a solid surface. The wall can be up to 60 feet long, 10 feet high, and 5 feet thick.

  • Wall of Stone

    A nonmagical wall of solid stone springs into existence at a point you choose within range. The wall is 6 inches thick and is composed of ten 10-foot- by-10-foot panels. Each panel must be contiguous with at least one other panel. Alternatively, you can create 10-foot-by-20-foot panels that are only 3 inches thick. [...] The wall can have any shape you desire, though it can't occupy the same space as a creature or object. The wall doesn't need to be vertical or rest on any firm foundation. It must, however, merge with and be solidly supported by existing stone. Thus, you can use this spell to bridge a chasm or create a ramp.

These spells all create walls but have highly specific and different rules for how those walls can be sculpted. Two can float in the air at whatever angle you like (Light and Force), the rest must be touching the ground or a particular material. Some are constructed with panels of a particular size (Force, Ice, Stone), while others have a set size (Light and Fire). Some must be a flat plane or a sphere (Force and Ice), while others infinitely shapeable (Stone) or can do cylindrical rings (Fire).

It's pretty clear from just these examples that the rules for shaping a spell, when available, are specific to the individual spell. What holds true for one spell does not hold true for others.

Conclusion

One of the core concepts of 5E is exception based design. If something is supposed to be different from the general rules, it will say so - and the explicit exception wins. The general rules are covered in Area of Effect, and make no provision for shaping.

As mentioned earlier, another core concept is use natural language. If it doesn't say it can be shaped, it cannot be shaped.

Specific Example

The specific example, Alarm...

Choose a door, a window, or an area within range that is no larger than a 20-foot cube.

One proposed answer to the linked question suggests you can break it up in blocks. The spell does not say a certain number of cubic feet, or panels, or a ring, or anything else. A room is an "area" in plan language, so Alarm fill a room's specific shape, as long as that room is "no larger than a 20-foot cube". If you're outside, it could cover a smaller area (like a clearing) as long as that area is "no larger than a 20-foot cube". But turning it into a ring (squared or otherwise) - nope.

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    \$\begingroup\$ one thing that makes me confused is the different but specific wording or spells like control water: " an area you choose that is a cube up to 100 feet on a side" move earth: "Choose an area of terrain no larger than 40 feet on a side" and others in the same vein. Why is the language different in this spell than those? Meaningful or oversight? \$\endgroup\$ – Rubiksmoose Jan 3 '18 at 21:15
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    \$\begingroup\$ Take it as it is, use natural language, don't try to apply logic or expect commonality in wording. The authors of each spell may not be the same person. \$\endgroup\$ – T.J.L. Jan 3 '18 at 21:17
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    \$\begingroup\$ @SeriousBri "20 foot cube" is an area of effect, it's one of the five basic ares listed in the first quote. \$\endgroup\$ – T.J.L. Jan 5 '18 at 16:26
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If you can choose an area, then it is not required to be 20ft cube shaped

Alarm says

Choose...an area that is no larger than a 20 foot cube.

If the area had to be a cube it would not have said to choose an area. Thus you must be able to shape it.

However...

All dimensions of the AOE must fit within a 20 foot cube

no larger than a 20 foot cube

This does not say "an area with a volume no greater than a 20 foot cube".

If you shaped this AOE to be a 5ft x 5ft x 320 ft area of effect it no longer fits inside a 20 foot cube. In plain English this would be referred to as being larger than the cube. Thus, it is larger than a 20 foot cube and is not valid.

So, no matter how you shape your AOE, it must fit within the constraints of a 20 foot cube.

This does mean that any doors or windows that would not fit into a 20 ft cube would not be able to be targeted (though you could still target some area within the doorway in a 20 foot cube).

If they had intended to allow you to shape it outside of the boundaries of a 20ft cube then they would have provided rules for it

Wall of Stone, Wall of Fire, and other spells like it all have explicit mechanics for how they can be shaped. Importantly, these all cite a minimum thickness.

The reason that is important in the case of Alarm is that there is no minimum thickness listed and thus it would be possible (if you ignored the 20ft cube limitation) to make the area infinitesimally small and incredibly long. Not only would this create an area unlike any other spell in the books, but it would also be fairly math heavy and difficult to adjudicate. It is almost certain that this was not at all the intention for this spell.

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5e is not meant to be an exercise in mathematics, geometry, physics, economics, etc. The oversimplification in 5e would heavily imply that the door/window/area must fit entirely within a 20' cube. If the spell could be shaped to alternate dimensions which would yield the same volume as a 20' cube it would explicitly say so. Spells such as Wall of Fire give precise instructions for how it can be shaped.

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    \$\begingroup\$ Oooh this is really getting deep into the interpretation but does the latter "that is no larger than a 20 foot cube" apply to door and window or just to the area. Because you certainly could have a window that is taller than 20 foot and quite possibly a great hall door which is larger. \$\endgroup\$ – Acts7Seven Jan 4 '18 at 15:39
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The real question you are asking is "what does it mean for one thing to be larger than another". That's not a clear concept in plain English. For example, what's the largest animal? Most people say it's the blue whale, but actually there are species of ribbon worm that are longer. Some people use that to say that the ribbon worm is the larger animal, but since it's only a bit longer and super thin most people say it's not.

So what is largeness? It's a subjective balancing of weight, volume, maximal extent, and possibly still more factors. It's true that 'no larger than X' with no other qualifiers implies you may choose any area meeting that qualification, including one shaped differently than X. It's not clear, however, whether a 20'X'1"X1" plank is larger than a 10'X3"X1" beam or vice versa.

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