Q: Do -1/-1 counters negate +1/+1 counters, or do they simply exist side by side? If I were to have a creature (lets say a 1/1) with two +1/+1 counters and add a -1/-1 would the creature have one +1/+1 counters with a -1/-1 counter making it a 2/2 with 2 +1/+1 counters to remove (if it had an ability that removed the +1/+1 counters to do something), or would I have a 2/2 with one +1/+1 counter to remove?
–Eric, Glassboro, NJ, USA
A: From the Magic Rules Corner:
Usually, R&D is careful to print only +1/+1 or -1/-1 counters within any given block, not both, and for a long time, -1/-1 counters were considered obsolete anyway. Shadowmoor features the return of -1/-1 counters, but it doesn't include any +1/+1 counters at all. Time Spiral, as usual, was another story, featuring throwback Spike Tiller alongside the "timeshifted" Giant Oyster—a situation that could arise around any casual table as well. In order to keep people from having to differentiate the two counter types on a single creature, a rule was introduced that would cause +1/+1 counters and -1/-1 counters to "cancel out" in the way you describe:
420.5n If a permanent has both a +1/+1 counter and a -1/-1 counter on it, N +1/+1 and N -1/-1 counters are removed from it, where N is the smaller of the number of +1/+1 and -1/-1 counters on it.
To put it another way, each +1/+1 counter cancels out a -1/-1 counter until only counters of a single type are left. This is a state-based effect—much like a creature with 0 toughness being put into the graveyard or a player with 0 or less life losing the game—which means that once the additional counters are put on, this canceling out happens before any player has a chance to do anything.
So, let's take a look at theoretical example about a 1/1 with an ability that removed +1/+1 counters—we'll use Twilight Drover.
Your Twilight Drover has two +1/+1 counters on it when some hypothetical Shadowmoor card puts a single -1/-1 counter on it. It now has two +1/+1 counters and one -1/-1 counter on it. Before any player receives priority, state-based effects are checked, and they "see" counters of both types on a single creature. One -1/-1 counter and one +1/+1 counter are removed, so one +1/+1 counter remains on the Drover that can be removed to pay for its ability.
However, it's important to note if a creature with both +1/+1 counters and -1/-1 counters has 0 or less toughness, it will be put into the graveyard immediately, with all of those counters still on it. This matters if an ability cares whether a creature had +1/+1 or -1/-1 counters on it when it died—for example, the Shadowmoor card Dusk Urchins cares about how many -1/-1 counters it had on it when it was put into a graveyard, and the Morningtide card Kinsbaile Borderguard does the same thing, but counts any counters:
Suppose Kinsbaile Borderguard has two +1/+1 counters on it when a hypothetical Shadowmoor card puts three -1/-1 counters on it. It is now a 0/0 creature with two +1/+1 counters and three -1/-1 counters on it. It's put into the graveyard as a state-based effect for having 0 or less toughness, but all of those counters are still on it. This means, in this case, that you will put five 1/1 Kithkin Soldier creature tokens into play.
Note also that none of this applies to the dizzying profusion of counters that were used before all power-and-toughness-altering counters were consolidated (+0/+1, +1/+0, +0/+2, +2/+0, +1/+2, +2/+2, -0/-1, -1/-0, -0/-2, and -2/-1). Only +1/+1 counters and -1/-1 counters cancel out this way.
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