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From many spells:

After 1 hour, or until you lose your concentration

This can be read either as "Whichever comes first" or "Whichever comes last". Presumably it means whichever comes first, but I can't find that stated anywhere explicitly and the ambiguity has come up here and also in my own games with the system.

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Is this actually different between the PHB and the Basic Rules PDF? For concentration spells, the Basic rules say something like "Duration: Concentration, up to 1 hour", which seems unambiguous. – Mark Bessey Aug 22 '14 at 3:26
@Mark Flipping through the PHB, everything I see uses the unambiguous wording. Possibly the other was found in earlier material? – mattdm Aug 23 '14 at 20:16
Just looked at the starter set rules; spells there also say "Concentration, up to some limit". – mattdm Aug 24 '14 at 1:49
up vote 21 down vote accepted

Whichever comes first.

PHB, page 203, Spellcasting, Duration, Concentration

If a spell must be maintained with concentration, that fact appears in its Duration entry, and the spell specifies how long you can concentrate on it.

The duration listed is the maximum allowed amount of time one can concentrate on that spell. In my interpretation, this is essentially saying that after 1 hour, you will fail any concentration check due to the intensity of the spell.

One possible ambiguity is when the magical act in question isn't explicitly a spell. The question you referenced is about an alchemical procedure, and the confusion there is over whether or not the time limit begins when the procedure is started or the procedure takes effect. There is no such ambiguity with spells, as the spells duration is measured from when the spell is completed and takes effect.

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That actually tells us a ton about the other answer too. You should answer that. Or edit that quote into the top answer. – wax eagle Aug 21 '14 at 15:55
Part of the issue regarding the other question (my own), is the addendum I added - When does the duration start, at the beginning or the completion? – SurrealAnalysis Aug 21 '14 at 15:58
@SevenSidedDie Good point, I added the ambiguity due to the other question they referenced, but that is not technically a spell, just a procedure. As far as I can tell, the duration of a ritual would start unambiguously after the ritual spell is done. – SurrealAnalysis Aug 21 '14 at 15:59

It's not ambiguous. Or means* just one part needs to be the case to satisfy the whole conjunction, so is one true? Yes? Then the conjunction is satisfied. Hence it is whichever comes first.

To make it mean whichever comes last, both conditions have to be true (one can't be last if the other hasn't happened yet), which is what the conjunction and commonly means.

Conveniently though, and unexpectedly since it's redundant and RPG rules often avoid that, the PHB makes this extra clear on page 203, saying that the time limit that appears is a limit on concentration to maintain the spell as well as the spell's duration in general.

* I'm not touching the rabbit hole that is debating exclusive or versus inclusive or. Suffice to say that it's not ambiguous in this case which or is being used.

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Shouldn't that be whichever comes last in the first paragraph? concentration | 1 hr evaluates true while concentration, or while 1 hr. So it would continue to be true if you concentrated longer than an hour. – wax eagle Aug 21 '14 at 16:25
@waxeagle It's not concentration | 1 hr, it's lost-concentration | 1 hr. But now that you mention it, misreading it like that may be the source of the confusion! – SevenSidedDie Aug 21 '14 at 16:30
To me the ambiguity is more: Is this when it ends, or how long it lasts? The former implies whichever comes first, whereas the latter implies whichever comes last. It’s especially confusing because duration normally means the latter, not the former. So the rule on page 203 is necessary to disambiguate. – Bradd Szonye Aug 21 '14 at 17:57
@BraddSzonye It is a duration, but the duration is itself described by two statements about ending: "after 1 hour, or until...". So it's the former, and whichever comes first. – SevenSidedDie Aug 21 '14 at 18:16
If it were xor, that would be great. Lose concentration on the spell exactly 1 hour after casting it and you have a permanencied spell. – Scott Aug 21 '14 at 22:23

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