Rule 107.1 · Numbers and Symbols

You can’t choose a fractional number, deal fractional damage, gain fractional life, and so on. If a spell or ability could generate a fractional number, the spell or ability will tell you whether to round up or down.

Most of the time, the Magic game uses only positive numbers and zero. You can’t choose a negative number, deal negative damage, gain negative life, and so on. However, it’s possible for a game value, such as a creature’s power, to be less than zero. If a calculation or comparison needs to use a negative value, it does so. If a calculation that would determine the result of an effect yields a negative number, zero is used instead, unless that effect doubles, triples, or sets to a specific value a player’s life total or the power and/or toughness of a creature or creature card.

Example: If a 3/4 creature gets -5/-0, it’s a -2/4 creature. It doesn’t assign damage in combat. Its total power and toughness is 2. Giving it +3/+0 would raise its power to 1.

Example: Viridian Joiner is a 1/2 creature with the ability “{T}: Add an amount of {G} equal to this creature’s power.” An effect gives it -2/-0, then its ability is activated. The ability adds no mana to your mana pool.

Example: Chameleon Colossus is a 4/4 creature with the ability “{2}{G}{G}: This creature gets +X/+X until end of turn, where X is its power.” An effect gives it -6/-0, then its ability is activated. It remains a -2/4 creature. It doesn’t become -4/2.

If a rule or ability instructs a player to choose “any number,” that player may choose any positive number or zero.