Does anyone have further comments? If not, I will write up specific errata/clarification notes based on what I said above, and post them for comment.
I still look with disfavour at the vague "inside the circle means inside the circle" definition, and I think the "cone or cylinder, degenerating to a flat disc when the circle is vertical" is better.
But if that vague definition stays, it should be complemented by a few examples that should give at least some intuition of how you think common borderline cases should be dealt with (e.g. circles lying on a vertical plane, circles lying on an inclined plane, stuff somewhat above or below the circle).
Let me finally add that some of this impacts Circles but also Boundaries and some open-roof Rooms/Structures (e.g. a walled garden is a Room: how high etc.?), so ideally the errata should manage to make the connection (unfortunately, I have no idea how to do it 
)
That actually has direct relevance to me since the entrance to our Covenant is in a walled garden that has an enchanted device with Range: Room observing it.
A prolate spheroid (capsule) based on the length of the circle. (Length of circle, square of length of circle, something like that.)
I honestly think something like this would be designed during spell creation and set as fact afterward.
Like a circle ward to keep a flame in and stuff for torches would have that info be a lot less vast than something like a warded circle against a demon or something.
In terms of play this would be an added descriptive sentence.
Basically, to me, one spell can have it a sphere and another a cylinder and another it could - with proper size modifiers - go up a mile and another down ten kilometers. It is a judgement call by the designer of the spell, but once set it is always there for that spell.
Honestly, I think the way to make people happy is something similar to the 'instant transportation' mechanics in Transforming Mythic Europe, where they said something along the lines of 'This is how it was designed to work, but your table can interpret it this other way without breaking things'.
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Here are my proposals for this thread.
First, errata Ring Duration: Ring (p. 112): Modify the first clause to read "until the target of the spell, or part of the target, moves outside the ring".
Second, errata Circle Target: Circle (p. 112): Add the following sentence as the third sentence of the definition. "Note that the circle itself is not inside the circle, and thus not targeted by the spell even if it would be a legal target."
Third, errata in the following insert (on page 113, although it wouldn't actually fit on there — this is a problem to be dealt with later).
Container Targets
Spells with "container" Targets, including Circle, Room, Structure, and Boundary, can work in one of two ways.
First, they can affect any valid target within the Target container at the time of casting, and continue to affect those targets even if they leave the original Target area, for as long as the spell lasts.
Second, they can affect any valid target within the Target container during the spell's period of effect. In this case, a valid target that leaves the container ceases to be affected by the spell, and a valid target that enters (or re-enters) the container is affected, until it leaves or the spell expires.
The way that a particular spell works is fixed when it is designed, and cannot be changed by the casting magus, although a spell working in one way is similar to a spell that is identical apart from working in the other, and so knowledge of one gives a bonus to inventing the other.
For example, a spell to put pink dots on people's foreheads with Target: Room and Duration: Moon could work in two ways. In the version that works in the first way, everyone in the room (on whom the spell Penetrates) at the time of casting gets a pink dot on their forehead, and this pink dot remains on their forehead until the new moon and full moon have both set. In the version that works in the second way, anyone who is inside the room (on whom the spell Penetrates) until the new moon and full moon have both set gets a pink dot on their forehead, even if they were not in the room when the spell was cast. When they leave the room, the pink dot disappears, although it reappears if they go back into the room. These two versions are two different spells, and a maga who wants to cast both (and cannot reliably cast them spontaneously) needs to create two spells.
Finally, make page 113 even more full by errataing in the following:
Circles and Rings in Three Dimensions
The standard Ars Magica rules use an intuitive definition of "inside the circle" for Circle Targets and Ring Durations. A person standing in the centre of a 1 pace diameter circle is inside the circle; someone who happens to be standing directly above it three floors higher is not. If your troupe need something more precise, you can work something out, and it should not break anything in the rules. Bear in mind, however, that it will still come down to the troupe's decision on whether a thing is within the more precisely defined volume.
Comments?
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Shouldn't Boundary Targets in Three Dimensions and courtyards targeted with T: Room be handled like Circles and Rings in Three Dimensions?
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Add:
Similar considerations apply to other container Targets if they are not closed in three dimensions, which is likely to be case for many Boundaries, and may be the case for Rooms or Structures.
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There are going to be a bunch of knock on effects from the change to "Container Targets".
This change allows a whole new way of obtaining spells and effects like the Semita Errabunda Fire Guardian (p. 4). If you change the initial effects duration to Sun, with the spell being the second type of container, then that is the only change needed to the original to make it a valid effect. You could shave off a use per day, so the Final level would be 29.
There are others, that one just jumped out at me since we just looked at it and you proposed an errata.
Do you really want to do this? This seems odd for two reasons:
- You seem to be making it akin to other "container" Targets. The structure itself is explicitly part of Structure.
- If the target of circular wards is the circle itself (the 2-D closed surface in the 3-D world), then it makes that much more sense that things cannot break the circle, let alone cross it.
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I agree with this. The Circle being the actual target makes more sense then the area contained for things like Wards. Otherwise how would the Circle Ward be used to trap something within it? The Circle being the actual target is the only way that it can function effectively against both internal and external objects.
Outside of the level shift, this actually fixes Fire Guardian. The effect is broken as written, as a fire isn't a "major magical features of the environment," and with Environmental Trigger an "item is only sensitive" to those. It may well be a major feature, but rarely if ever a major magical one. Making it constant with Sun, Environmental Trigger, and 2/day would solve this, raising the level by 9. Otherwise you need an Intellego effect.
I agree with the folks above. At the very least it needs a “wards work differently” sort of statement but Callen has a point about if we want similarity between container targets the circle should be included.
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Sure. It seems quite right that if I turn everyone inside of a house into frogs, they should stay that way if they hop out; similarly for Target Circle (so long as the spell is not Duration Ring).
I'm asking for comments to settle precisely that question.
Both your points are good, so I will delete the erratum. I am a little nervous about someone coming up with stacked Ring/Circle spells that do something horribly game-breaking forever, but the clarifications to Ring Duration should block most of them.
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Excluding the Circle itlself won't fix those; they are inherent in the "permanent" nature of Ring.
It would only make the setup a little more "fragile", and if wards are explicitly made "more robust" even that mostly falls.
I'm all for "the circle falls within the circle". It's so much cleaner.
Ring/Circle spells protecting against everything of any Hermetic Form from the outside should still work. How to break such a Ring/Circle, if it is protected by these spells gainst stuff of all Hermetic Forms?
Anyway, a request of equal treatment of Circle, Room, Structure and Boundary implies rewriting many spells and likely reworking the game world.
That is a vast exaggeration (and things go in the opposite direction, in fact), in my opinion.
Whereas an equal treatment of "Container" spells makes ArM5, a mechanically-complex game, more accessible. This is really important: most players do not have the same level of rule-mastery as the average person discussing these threads. Keep it simple (and mythic)!
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I cannot agree more !