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There are plenty of cards in MTG that have effects that allow you to cast cards for free, such as Aluren and Omniscience. These cards phrase their abilities as

You may cast spells from your hand without paying their mana costs.

There are obvious differences in what cards are free, like the aforementioned Aluren that only allows creatures of converted mana cost 3 or less, but the cards all contain some form of the phrase "without paying their mana costs".

Contrast this with Rooftop Storm, which reads

You may pay 0 rather than pay the mana cost for Zombie creature spells you cast.

Now obviously, if you ignore the restrictions on the three cards I've mentioned, they all allow you to cast certain cards for free. But why does Rooftop Storm phrase its ability in this way? Are they functionally different in certain situations?

  • Rule 118.9. suggests they are the same: Some spells have alternative costs. An alternative cost is a cost listed in a spell’s text, or applied to it from another effect, that its controller may pay rather than paying the spell’s mana cost. Alternative costs are usually phrased, “You may [action] rather than pay [this object’s] mana cost,” or “You may cast [this object] without paying its mana cost.” I'm not sure if that is the entire story, though. – Glorfindel Nov 27 '19 at 18:01
  • I don't see any difference, and googling didn't find any either. (One thread suggested the latter wording is used for static abilities and the former wording is used elsewhere, but that's obviously not true.) – ikegami Nov 27 '19 at 18:47
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These effects are functionally equivalent. In all cases they allow you to cast certain cards with alternative costs of no mana instead of their normal mana cost.

Rule 118.9 says this:

Some spells have alternative costs. An alternative cost is a cost listed in a spell’s text, or applied to it from another effect, that its controller may pay rather than paying the spell’s mana cost. Alternative costs are usually phrased, “You may [action] rather than pay [this object’s] mana cost,” or “You may cast [this object] without paying its mana cost.” Note that some alternative costs are listed in keywords; see rule 702.

Rooftop Storm uses the first template, and Omniscience and Aluren use the second.

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  • Re "There isn't a good way of writing that with the Omniscience template.", "You may cast Zombie creature spells without paying their mana costs." – ikegami Nov 27 '19 at 18:52
  • You're right, that didn't make sense – murgatroid99 Nov 27 '19 at 18:54
  • But I think you're right nonetheless. Confusingly, "you may cast" is sometimes an instruction to cast ("you may cast the revealed card without paying its mana cost") and sometimes a replacement effect that affects what/when you can cast. ("You may cast spells from your hand without paying their mana costs.") (It took me a while to really understand that when I first started.) – ikegami Nov 27 '19 at 19:02
  • What follows "you may cast" implies what those words mean. And by removing "from your hand", it removes a hint that RS is a replacement effect rather than an instruction. It still has to be a replacement effect with the wording I mentioned, but it would be deviating from the standard template in an already-confusing situation. The wording RS uses removes the confusion. – ikegami Nov 27 '19 at 19:02
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The ruling on Omniscience states:

If a spell has Variable Colorless in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.

This implies that the alternative casting method allowed by Omniscience is identical to casting with a cost of 0, which is what Rooftop Storm does. Trinisphere still functions on cards cast with Omniscience.

It seems like the difference in wording is mere flavor, and not the difference between cost=0 and cost=null.

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  • That ruling is just based on the much more general rule that when using an alternate cost that does not include {X}, the value of X must be 0. That doesn't necessarily imply that these two alternative costs are the same. It turns out that they are the same, but the reasoning here doesn't actually show that. – murgatroid99 Nov 27 '19 at 18:34

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