121.1.
A counter is a marker placed on an object or player that modifies its characteristics and/or
interacts with a rule, ability, or effect. Counters are not objects and have no characteristics. Notably,
a counter is not a token, and a token is not a counter. Counters with the same name or description
are interchangeable.
121.1a
A +X/+Y counter on a creature or on a creature card in a zone other than the battlefield,
where X and Y are numbers, adds X to that object‘s power and Y to that object‘s toughness.
Similarly, -X/-Y counters subtract from power and toughness. See rule 613.3.
121.1b
The number of loyalty counters on a planeswalker on the battlefield indicates how much
loyalty it has. A planeswalker with 0 loyalty is put into its owner‘s graveyard as a state-based
action. See rule 704.
121.1c
. If a player has ten or more poison counters, he or she loses the game as a state-based action.
See rule 704. A player is «poisoned» if he or she has one or more poison counters. (See rule 810
for additional rules for Two-Headed Giant games.)
121.2.
Counters on an object are not retained if that object moves from one zone to another. The
counters are not «removed»; they simply cease to exist. See rule 400.7.
121.3.
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 as a state-based action, where N is the smaller of the number of +1/+1 and -1/-1
counters on it. See rule 704.
121.4.
If a permanent with an ability that says it can‘t have more than N counters of a certain kind on it
has more than N counters of that kind on it, all but N of those counters are removed from it as a
state-based action. See rule 704.
121.5.
If an effect says to «move» a counter, it means to take that counter from the object it‘s currently
on and put it onto a second object. If the first and second objects are the same object, nothing
happens. If the first object has no counters, nothing happens; the second object doesn‘t get a counter
put on it. If the second object (or any possible second objects) is no longer in the correct zone when
the effect would move the counter, nothing happens; a counter isn‘t removed from the first object.
121.6.
If a spell or ability refers to a counter being «placed» on a permanent, it means putting a counter
on that permanent while it‘s on the battlefield, or that permanent entering the battlefield with a
counter on it as the result of a replacement effect (see rule 614.1c).