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My friends and I are relatively new to MtG. At some point, one of them introduced us to the idea of mana weaving, and we'd do this to declump our decks prior to a very thorough shuffling. However, at the Theros pre-release, one of us was asked not to do this, and we learned it was frowned upon and ceased to do it.

I just now came across Shuffling Do's & Don'ts, and it sounds like mana weaving's only bad if you use it to leave your deck in an even distribution, and it's OK if your deck is then thoroughly shuffled. In hindsight, the judge may have simply thought my friend wouldn't have shuffled afterwards, but at the time I came under the impression it was frowned upon even if you did shuffle afterwards.

So: is mana weaving my deck OK, if I shuffle it very thoroughly afterwards?

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What is the point of it, if you then do a proper shuffle? –  TimLymington Jan 10 at 0:01
@Tim Declumping, mainly. But I'm not sure - my friend was stopped even though he was going to shuffle afterwards, but maybe the judge thought he wouldn't. –  Jonathan Hobbs Jan 10 at 0:11

3 Answers 3

up vote 13 down vote accepted

Look at it this way:

  • If you properly randomize your deck, then its initial configuration is irrelevant. So "mana weaving" is just wasting more productive time.

  • If you don't properly randomize your deck, then "mana weaving" is likely part of you cheating.

So, there's no upside. "Mana weaving" doesn't actually accomplish anything, unless you're accidentally or purposefully cheating.

Judges routinely prohibit "mana weaving" because the down side of disrupting someone's pointless pre-game ritual is much smaller than the upside of guaranteeing a fairer game.

Some players "mana weave" to "break up clumps." That's based on a mistaken assumption. A truly randomized deck isn't one where the lands and spells are perfectly distributed; it's one where all the cards are in an arbitrary and unpredictable order. That'll mean you sometimes get lucky and sometimes you won't. Bad draws can be frustrating, but some amount of clumping — even the rare incident of really egregious clumping — is perfectly normal. A player who never gets mana-flooded or mana-screwed isn't shuffling properly.

If you're constantly experiencing clumps, especially drawing cards in an order similar to how you played them last game, then it's a sign you're direly under-shuffling. Spend more time shuffling (2-3 minutes before a game is pretty standard) and practice stronger techniques like riffle or mash shuffling.

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+1. This is exactly what I've heard judges say. –  ikegami Jan 10 at 4:57
Re: under-shuffling -- Based on the paper by Bayer & Diaconis, a 40-card (Limited) deck should be shuffled 8 times. A 60-card (Constructed) deck should be shuffled 9 times. A 100-card (Commander) deck should be shuffled 10 times. (More generally, 3/2 * Log_2(n) shuffles, where n is the number of cards in the deck.) Shuffling more isn't harmful, but does not increase the variance by a sufficiently significant amount to warrant it. (Of course, that's assuming a "perfect" riffle; poor shuffling would require more iterations.) –  Brian S Jan 10 at 6:27
@BrianS Didn't go back to the paper, but recalling a Diaconis lecture I attended, this assumes a near-perfect but not-quite-perfect riffle shuffle. With a 52-card deck, as Diaconis demonstrated, 7 perfect riffle shuffles will exactly reset the deck. Small deviations from perfect shuffles, however, will propagate through the deck in unpredictable ways--which is what you want. –  Gregor Jan 10 at 10:35

The elephant in the room here is probably that it's kind of hard to really thoroughly shuffle a deck.

Around 12 riffle shuffles will do it for a 60-card deck, with a few more needed for bigger decks. (The complicated expression given in the linked paper is pretty well approximated by 2·log2(n), so doubling the deck size requires two more riffles to achieve thorough randomization.) With three minutes to shuffle the deck between games, that means up to 15 seconds per riffle, which is quite doable with a bit of practice.

But of course, riffle shuffles are kind of hard on cards, so maybe you don't want to subject your deck to a dozen of them before every game. That means resorting to slower and/or less efficient shuffles like the mash (which is actually pretty close to a riffle), the wash (slow and messy, but OK if done well) or even the pile (slow and potentially cheaty) or overhand (just plain lousy) shuffles.

So the upshot of all this is that, unless you shuffle your deck very thoroughly (say, a dozen riffle / mash shuffles) before each game, some traces of the original non-random order are going to remain. The question, then, is whether that non-randomness ends up helping you or harming you — and if you start with a mana-woven deck, it'll probably do more to help than to harm.

In general, as long as everyone starts with a similarly ordered deck and shuffles it equally well, it doesn't really matter if the shuffle is a tiny bit short of perfect, since any advantages or disadvantages due to the imperfect shuffling will be the same for everyone. But if you start with a mana-woven deck and your opponent starts with a non-woven one, it could give you an unfair advantage.

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It's really easy to mash shuffle 12 times in 3 minutes. –  murgatroid99 Jan 10 at 7:26
"pile shuffle" is not a shuffling technique. It should not be used in competitive Magic except potentially as a means to count the number of cards in the deck. –  Brian S Jan 10 at 16:53

If we assume that when you say that you shuffle thoroughly, you mean that you completely randomize the order of your deck, then it should be fine.

If you put your deck in any particular order, and then completely randomize it, then the original order doesn't matter and your deck is properly shuffled.

If you put your deck in a certain order and then "shuffle" in a special way so that your deck ends up in the order you want, then you have stacked your deck, which is not allowed.

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