agree to disagree

As far as I'm concerned, you can functionally achieve the desired result at any resting locations using alarm (or any of the other example spells) as written targeting the area where the cart is parked whenever it is stopped. Just like a real car alarm, it isn't on while you're driving it, you can probably notice intruders then. You arm the car alarm when you're not going to be watching it for a while. It makes a little more work, but it could be done by an appropriately high level wizard (or a custom magic item that has a few charges of each of those spells and allows only 1 programmed illusion to be active at a time).

I also do agree that using alarm on a wagon is a reasonable use that isn't far from RAI, even if it isn't RAW. A lenient DM might let you get away with it or homebrew a spell based on alarm that replaces the word "area" with "vehicle" called "Car Alarm." 

I also don’t like moving the goalposts. Vehicles use movement rules, though it is not in a per round movement speed. Nonetheless, it seems an arbitrary distinction to say that some objects on the tables of vehicles count as areas whereas others don’t purely by one persons feelings on the subject. A wagon might be 8’x12’ or larger. Certainly large enough to draw a small battle map of it. Either an area cannot move (and spells that target areas do not move with a vehicle) OR areas can move. The one possibility that seems least mechanical is to have it both ways arbitrarily (that is, with no rules helping to make the distinction). 

If the "area" referred to by the spell description had to be on the ground, it would say "area on the ground" instead of just "area".

If it couldn't move, the spell text would say so, as in Glyph of Warding.

At least *one* of these things would be different if Alarm would not work on a cart.

Glyph of Warding says "can't move" because its target is a surface or object, and surfaces and objects move all the time. Alarm doesn't say its area can't move, because there are no areas that can move other than those that say they can move. Areas not moving has nothing to do with whether they're an area of ground or an area of space, it has to do with the plain-language meaning of "area." An area is a place, and places stay put. Alarm does not say that its area can move, and does not say that its objects (door/window) can't, so we're left with:

1) the door can move

2) the area can't. 

From where do we get the absolute rule that an area is a place? You say it's a place and places stay put, and as far as I can tell you're citing your own opinion as proof. By plain language, the *area* of the bed of a cart can certainly fit within a 20 foot cube. Certainly an *area* of the bed of a cart can fit within the total *area* of the cart.

"Alarm doesn't say its area can't move, because there are no areas that can move other than those that say they can move." I'm genuinely interested in seeing an example of an area that you say can move.

There are areas on carts. There are areas on boats. The first class section on an airplane is an area - the area where people who paid extra can sit and I can't. Flight attendants, if possible, might even want to set up an alarm just in case I try to leave the area I am in and enter the restricted area.

Quote from TaynofDarkwood:

Everything is moving. The planet is moving. Tectonic plates are moving. 

HAHAHAHAHAHAHAHAHA...... ok.... i don't gonna make the easy joke here... but, it's obvious ... heh ??

We have two choices for how to read the mobility of areas: either areas cannot move (all areas unless they say otherwise, not just this spell), or areas can move (all areas unless they say otherwise, not just this spell). Let's take a look at each:

Option A: Can't Move

• Ludicrous results? 
  
  • For the Alarm spell, can ward a bedroom in a house but not a cabin on a boat or a moving wagon. Also, like, the whole universe is moving when you really stop to think about it, mannnnn *deep hit*
  • Other than that, none. 
• Desirable results?
  • It's literally the easiest thing in the world. If you pick an area on a battlemap to cast a spell, it stays in the spot that you designated unless the spell or ability specifically provides otherwise (such as Cloudkill or Dawn, or an area which is "centered" on a moveable creature or object which does not prohibit the movement of such area, such as Antilife Shell, Aura of LIfe, the second targeting option for Darkness, etc.). 
  • Will feel intuitive to most groups, since this is the way that the rule actually works and has always worked for every group everywhere outside of this single thread of conversation. 

Option B: Can Move

• Ludicrous results?
  • Spells like Black Tentacles start slithering around all over the battlefield. How far/fast can they move? Unknown, since the spell doesn't actually say they can, so we're well off the deep end of just making stuff up! Create Bonfire, Darkness (specifically even when it's not cast on a moveable object like the spell specifies), Earthquake, Hallucinatory Terrain, etc etc. Hell, Druid Grove can be hauled around in a wagon full of dirt now, and the "same area" is anywhere you park every night! Fantastic!
  • If areas can move, why not "points" too? Cloud of Daggers gets to go whizzing around too! Why not anything and everything? Doors can move, why not let the party pick up the Demiplane door they created and carry it with them on the Fighter's back? Is there even such a thing as a static effect spell any more? No, probably not, unless the spell goes out of its way to specifically say it can't be moved like Glyph of Warding?). 
• Desirable results?
  • I mean I guess you can cast Alarm in your boat cabin now. Nothing else makes any sense any more, but at least you'll always have that? 

For option A ludicrous results, you forgot to also include all of the things that you have listed as option B ludicrous results whenever cast on a moving vehicle. If they are ludicrous in one situation they are equally so in the other, like when those black tentacles just start sliding off the deck of your pirate ship mid battle.

You can't say that it is sometimes option A except when you feel like option B makes more sense (especially when there aren't any rules in the texts to help us decide).

I would posit that option C exists: You can choose an area or point that is relative to something that is relevant and in the vicinity that you would call the ground or floor.

• Option C has exactly as much mechanical support from the text as options A and B and provides:
• Desirable results:
  • Every spell works the way you'd expect in almost all situations.
  • Your black tentacles stay where you put them on dry land and on your ship at sea.
• Ludicrous results:
  • You can cast alarm on a WAGON?!?! Oh my goodness! That's broken as sin.

Look man. Points don't move, unless spells say you can. Areas dont move, unless spells say you can. Most DMs I've encountered don't think that "ground" includes ship decks, second stories of buildings, etc etc. The simple reading is the RAW reading, and you're all twisted up trying to houserule something to solve a problem that you've created out of thin air yourself for the grand purpose of.... casting alarm on a moving wagon. 

Areas don't move unless they say they can, it's just the way it is. But you live your life in whatever special homebrew way you want, unsubscribed and OUT. 

Look man, my option allows spells to work the way you want them to in most cases but allows things with floors that move with respect to the ground or a fixed point to still behave as you'd probably expect. If that is too much for you, I'm sorry. And it is an option for something that is all up to DM adjudication anyway, since no definition of ground is given in the books.

Edit: and as a final point (that you won't see anyway since you're unsubbed) I would just like to point out that we have both argued on either side. Your original position was that you could cast these spells on a boat of sufficient size, without telling us what was sufficient. Options A, B, and even C are all possibilities and it is up to the DM to decide which to use. C provides most of the benefits of both: you cannot move areas around at the drop of a hat, they all still move relative to a fixed reference frame. You can't move tentacles around on your boat, but they still stay on your boat. The other thing that option C has that your original statement didn't is a(n) (admittedly invented) guideline that tells you when you can use what frames of reference.

Thanks everyone for the input.  I didnt think there would be so many different views!  
i accept that Alarm would not likely work as i thought it might.  One question though... would it make any difference if i never intended for the spell to “work” while i was actively moving?       I dont have access to the spells i listed and was planning on paying someone to cast them on the wagon for me.  My thought was that the Alarm spell would be in place to “work” whenever i stopped the wagon.  I didnt expect, or even want it to be active while moving, only when stopped.  That was what i meant by “travelling with the wagon”.  

But now that i have actually written that out i see the flaws!!!

thanks again.

I think that the way I'd probably handle what you describe in one of the ways that I mentioned in my first post: make the wagon (or some object you keep in it) a custom magic item that has charges that refresh daily of the spells that you need and a limitation that only 1 of each spell can be active at a time. Otherwise, alarm has a short enough duration that you won't be able to have a wizard cast it once and make it useful for any length of time.

you are absolutely right Wolf, i forgot Alarm was only 8 hours.  It was the others that i believe were “until dispelled”.  Maybe i’ll just go with a couple of big dogs!!

Unsubscribed, I know, but wow, talk about moving the goalposts. Evard's, etc. don't specify an area. They specify points on the ground or natural terrain or some other specific location that does not move. Alarm specifies an area, not a point on the ground, not even an area you can see! These requirements are all spelled out in other spells, but we're supposed to just infer that they apply to this one? That's wacky. It's not like they included them in the other spell descriptions but left them out of Alarm to save ink or something.

You're referring to RAW without citing any written rules, just your say-so.

And why on earth would it be broken to protect a cart? Or a cabin on a boat?

Not subscribed, but that don't mean I don't lurk now and again :)

Dndbeyond doesn't' have a way to sort "spells that target areas" (such as Hallucinatory Terrain) versus "spells that create areas around a targeted point/ground/object/creature" (such as Black Tentacles, Cloud of Daggers, or Aura of Life) versus "spells that do some combination of the above" (such as Alarm, or Darkness). When posting, I was just going through the spell list grabbing area spells with duration, not picking those specifically. 

I do agree that there is something intrinsically different about the various sub-categories: it seems plain-English to me that objects/creatures are allowed to move unless a spell specifies otherwise, but points and "ground" are fixed coordinates, as are areas not linked to any target other than themselves. I'll agree that it isn't defined RAW that way, but it does fit the normal conventional use and understanding of those words... and there's other textual support for that interpretation.

Spells creating areas which target objects, and especially those that target creatures, seem to all describe what happens when that creature moves. For Darkness, "the darkness emanates from the object and moves with it." For Aura of LIfe, "the aura moves with you, centered on you." Not sure if every such spell says that, I haven't read every spell, but it is immediately logical that a player would ask "what happens if the target moves?", since targets can generally move and there are rules found elsewhere describing how that movement generally happens (creatures walk, objects get carried or thrown or whatever, etc). 

Spells that create areas around points or ground don't seem to describe what would happen if that point or ground were to somehow move. For Cloud of Daggers, all we know is that the area is "centered on a point you choose within range." For Black Tentacles, we have an effect on "a 20-foot square on ground that you can see within range. For the duration, these tentacles turn the ground in the area into difficult terrain." There's no description of what happens if that point or if that ground were to move... and while one interpretation could be "well it doesn't say points or ground CAN'T move, so they can!", I think it much more reasonable to say "my normal English-language understanding of 'point' and 'ground' is that they are fixed in place, and it is reasonable to think that the rules are using those terms the same way unless they give me a reason to think otherwise." 5E doesn't really have a general system to help me understand how or when a caster might move a point after its been created, or how points or ground might move on their own, and it really just opens up too many unanswered questions that will require me to start creating more houserules if I entertain those as possibilities. Those that break that mold and do move, like Cloudkill, tell you how they move.

Spells that create areas without a target are closer to point/ground spells than they are to creature/object spells. For the 20-foot-cube option of Alarm, all we know is that you choose "an area within range that is no larger than a 20-foot cube" and "an alarm alerts you whenever a Tiny or larger creature touches or enters the warded area." It doesn't address what happens what happens if the area moves... because how would an area move? If you want to start drawing new houserules around "well, you could define a discrete area within a moving object, and the area would move in relation to other things outside of that object...", where do you start drawing lines around how contained that area has to be (just enclosed carts? open wagon beds? the 20 foot cube whose corner originates in this single cup?)? If you can link an area to an enclosed object, would you also allow an area to be described as being centered on an object like a stone or a weapon, when the spell doesn't provide that it is centered on an object (that was Sigred's reading of the spell in the first place, after all)?

It's not that any of these are broken, of course there would be no problem with an alarm spell that followed your wagon wherever it went. But the spell isn't written that way, and taking the position that "I can do anything that the rules don't tell me I can't" results in  needing to draw your own lines around how far you want that to go which goes off the rails of RAW pretty quickly. It also renders a bunch of language meaningless in other spells, because if any area spell can be pinned to an object  or creature and move with it, then why do some spells bother to waste time telling you that you can? 

In closing... as I stated earlier in this thread, personally, I'm fine with treating ships as non-moving when the story is taking place on a ship, if the party isn't going to go insane trying to rush down a slippery slope of implications. Sure a wagon and a ship are both vehicles, but one is a setting, and one is a piece of equipment that moves through scenes, and that's why I as a DM would allow Alarm to be cast on a ship cabin but not a stagecoach. Other DMs are free to draw that line wherever they want. But the RAWest rule with the brightest lines is: if something is measurably moving like a wagon or a ship or whatever else, an "area" can't hitch a ride but rather slides right off the back of it as it moves. It's the natural implication of reading all of the spells as written without coming up with new systems, because all the systems to arbitrate it are already in place (draw where the spell was cast, move other objects around/through it as they move under the normal rules for their movement). 

This may cause more problems then it solves but here it goes.

Let’s say you have a floating island. If the island is stationary, no problem, but if it’s moving it would drift away from certain spells and spells like glyph of warding would be broken.

I see the solution as defining a frame of reference when you cast the spell. If the frame of reference moves, the spell moves with it. If the spell would normally effect an area larger than the frame of reference, the area is limited to the size of the frame of reference.

For the floating island, most spells would perform normally. For a wagon, the spells would work normally within the wagon and would move with it but wouldn’t effect anything outside the wagon.

For glyph of warding, you would need to establish a minimum size for the frame of reference otherwise you could make the frame of reference a chest and have the warded book in the chest and move it wherever you want.

If you're wanting to pin the effect to a frame of reference as a way to resolve all of this, I would suggest that frame of reference be "the ground beneath your area" instead of letting the player define an object or (semi?)enclosed space as that reference. The only way "ground" moves is in your floating island example, a land-slide, sink-hole, or a fissure opening, so the opportunities for shenanigans and legalese exploitation are minimal. 

I agree with you that is the simplest solution that doesn’t allow game breaking shenanigans and that’s probably what most DMs really want.

But I keep coming back to a scenario inspired by the Stallone/Schwarznegger movie Escape Plan. (Warning Spoiler: Really? How could they not know they are on a ship? Of course maybe they did but the audience didn’t.)

Caster is in a room in a ship. Casts Light in the center of the room. As the ship moves, the light drifts away. It would be a wonderful navigation tool especially if you were relying on dead reckoning. 

Maybe it’s just the scientist in me that prefers the Relativity solution. 

Light is cast on an object and creates light in a radius around that object wherever it moves. Darkness can be cast that way on an object that can move, or can instead be cast on a fixed point in space that would act like you're describing, 'drifting away' as the boat moves. There is no need to depart from the rules text of these spells to avoid absurd results.