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The description of the Light spell in D&D 5e says:

You touch one object that is no larger than 10 feet in any dimension. Until the spell ends, the object sheds bright light in a 20-foot radius and dim light for an additional 20 feet. (...)

Can I cast the Light cantrip on an 11-foot pole?

On one hand, 11 is greater than 10. This would suggest that the answer to my question is "no".

On the other hand, I could put an 11-foot pole in a 10×10×10-foot box. This would suggest that the answer is "yes".

I don't expect that the success of an adventure will hinge on the answer to this question any time soon, but I would like to understand how dimensions are supposed to work.

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No, you can't cast light on it.

If the spell intended to say "an object that fits within a 10-foot cube", it would say that, or something like it. The phrase "no larger than 10 feet in any dimension" is very clear: the object can't be more than 10 feet in its longest possible measure. You don't get to pick an arbitrary X-, Y-, and Z-axis and only measure against those three.

This does leave a hole in the rules where you cast light on an object that is less than 10 feet long, and then change it so that it is longer - for example, casting it on a ball of yarn that you then unroll, or a telescoping pole that you then extend. I would probably rule that when the object expands past 10 feet, it becomes an invalid target and the spell fizzles out, but that's just my ruling.

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  • \$\begingroup\$ Comments are not for extended discussion; feel free to continue in chat. \$\endgroup\$
    – Someone_Evil
    Jun 21 at 20:09
  • \$\begingroup\$ I guess one could rule that just the first 10 feet of the object are shining bright. \$\endgroup\$ Jun 22 at 22:01
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RAW: No, you can't cast light on an 11-foot-long pole.

It appears to be pretty straightforward. The description of the light cantrip says:

You touch one object that is no larger than 10 feet in any dimension.

An 11-foot-long pole is greater than 10 feet in one of its dimensions, so it is not a valid target. The spell description specifically says "dimension" (length, width, depth), and not "volume" or whether it would fit in a 10-foot square.

That said, it is hardly balance-breaking if a DM were to allow it.

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    \$\begingroup\$ Can you cast the spell on a 10x10x10 cube? \$\endgroup\$ Jun 20 at 14:54
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    \$\begingroup\$ Where does the spell description specifically list the dimensions as length, width, and depth? My phb doesn't mention that. \$\endgroup\$ Jun 20 at 15:17
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    \$\begingroup\$ @indigochild The PHB doesn't define "dimensions", but from Wictionary: "A measure of spatial extent in a particular direction, such as height, width or breadth, or depth." \$\endgroup\$
    – Wanderer
    Jun 20 at 21:46
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    \$\begingroup\$ @ThomasMarkov no, because the 10x10x10 cube has at least one dimension that is longer than 10ft: its longest being the diagonal cutting across the opposite corners of the cube (10√3, ~17 ft). \$\endgroup\$
    – asgallant
    Jun 21 at 20:54
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    \$\begingroup\$ So the largest item you can cast it on is a sphere with a 10ft diameter. \$\endgroup\$ Jun 21 at 22:18
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No, but you can cast it on the 10' cubic box the pole fits in

This is an interesting puzzle you present. It is clear, as you say, that an 11 foot pole is not a legitimate target for a light spell. And yet, you assume, a 10 x 10 x 10 box is a legitimate target for the spell. Since the pole will clearly fit inside the box, in some sense the box is larger. How can the larger box not be too big while the smaller pole is too big?

Like Light, a number of spells in the PHB target objects of limited size: Drawmij's Instant Summons ("an object weighing 10 pounds or less whose longest dimension is 6 feet or less"), Mending (a "break or tear [that] is no larger than 1 foot in any dimension"), Nondetection ("a willing creature or a place or an object no larger than 10 feet in any dimension"), Stone shape ("a stone object of Medium size or smaller or a section of stone no more than 5 feet in any dimension"), and Wish (an "object [of] no more than 300 feet in any dimension").

Referencing these descriptions themselves only, it is not clear what "any dimension" means. A number of commenters (on other answers and earlier versions of this answer) have pointed out that "any dimension" can include the 'space diagonal' of a three dimensional object, which is longer than any of its sides. For example, a 10' x 10' x 10' box has a face diagonal of over 14 feet on each surface and a space diagonal of over 17 feet running through its interior. Thus, they reason that a 10' cube is not a legitimate target for a light spell, since it has an interior dimension of nearly double ten feet.

However, two sections of the PHB make it clear "how dimensions are supposed to work": dimension does NOT include diagonals.

Move Earth says

Choose an area of terrain no larger than 40 feet on a side within range. You can reshape dirt, sand, or clay in the area in any manner you choose for the duration. You can raise or lower the area’s elevation, create or fill in a trench, erect or flatten a wall, or form a pillar. The extent of any such changes can’t exceed half the area’s largest dimension. So, if you affect a 40-foot square, you can create a pillar up to 20 feet high, raise or lower the square’s elevation by up to 20 feet, dig a trench up to 20 feet deep, and so on.

If the "area's largest dimension" were allowed to be the diagonal, you could lower the elevation by more than 28 feet. By stating that the maximum change is 20 feet, the spell is clearly indicating that only the "sides" (and not the diagonal) count as its largest dimension.

The Monk Disciple of the Elements Class Feature Shape the Flowing River says

As an action, you can spend 1 ki point to choose an area of ice or water no larger than 30 feet on a side within 120 feet of you. You can change water to ice within the area and vice versa, and you can reshape ice in the area in any manner you choose. You can raise or lower the ice’s elevation, create or fill in a trench, erect or flatten a wall, or form a pillar. The extent of any such changes can’t exceed half the area’s largest dimension. For example, if you affect a 30-foot square, you can create a pillar up to 15 feet high, raise or lower the square’s elevation by up to 15 feet, dig a trench up to 15 feet deep, and so on. You can’t shape the ice to trap or injure a creature in the area.

As with move earth, the description of this ability makes it clear that the "largest dimension" can be either length or width measured along the surface, but not an interior diagonal.

Thus, an 11 foot pole is not a valid target of the light spell because its largest dimension, its length, is longer than 10 feet. However, such a pole can easily fit within a 10' x 10' x 10' box. By choosing the dimensions of face diagonals and space diagonals, it is clear that some dimensions of the box are larger than 10 feet. However, according to the way the PHB indicates that dimensions work, these diagonal dimensions do not count for the application of the size limit of the spell, merely its exterior length, width, and height.

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  • \$\begingroup\$ Comments are not for extended discussion; this conversation has been moved to chat. \$\endgroup\$
    – V2Blast
    Jun 21 at 22:34
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    \$\begingroup\$ Wait, if I'm understanding correctly, this means that a 19-foot-tall icosohedron is a valid target? \$\endgroup\$
    – tuskiomi
    Jun 22 at 22:15
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    \$\begingroup\$ this doesn't feel right. even if a box is hollow it has dimensions larger than 10 feet along its three dimensional diagonal the wording "that is no larger than 10 feet in any dimension" should also be valid for the diagonal that has the 'hollow' part in between and that is larger than 10 feet. If you allow hollow parts, then you any wire frame structure would have holes and would be allowed to exceed the 10 feet \$\endgroup\$
    – gelonida
    Jun 23 at 9:13
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    \$\begingroup\$ @tuskiomi After finding context in another spell and ability in the PHB, I have substantially edited my answer - you might want to read the new version. To your point, I would now assume that the 19 foot height would render that target illegitimate. \$\endgroup\$
    – Kirt
    Jun 26 at 17:00
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    \$\begingroup\$ Yeah the reasoning w/o hollowness is much more coherent. However I still interpret the phrase no larger than x feet in any dimension to mean, the worst case dimension. The object had to be rotated to reveal its worst case size. Thus with my understanding the biggest object that could be affected would be a sphere with 10 feet diameter. The biggest cube would be a cube with a diagonal (in 3 dimensions) having 10 feet which means a 5.77 x 5.77 * 5.77 foot cube. However I didn't check so far not whether rules with the above phrasing give examples violating my assumption \$\endgroup\$
    – gelonida
    Jun 28 at 12:12
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The other answers are correct.

This is where the misunderstanding lies.

On the other hand, I could put an 11-foot pole in a 10×10×10-foot box. This would suggest that the answer is "yes".

What seems to be missed here is that the box itself is larger than 10 feet in some dimension. At its shortest dimensions (the x, y and z axis), the box is indeed exactly 10 feet. But at its longest, opposite corners to opposite corners, the box is much longer than 10 feet. In fact, the box is the square root of 300 feet ie. about 17.32 feet which you can get from the Pythagorean Theorem.

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    \$\begingroup\$ There’s a neat theorem here, for an n-dimensional unit cube, the length of the longest diagonal is always sqrt(n). \$\endgroup\$ Jun 23 at 11:13

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