By default, effects that cause a player to discard a card allow the affected player to choose which card to discard. Some effects, however, require a random discard or allow another player to choose which card is discarded.
May 3, 2019 · War of the Spark
To double a creature’s power, that creature gets +X/+0, where X is that creature’s power as the spell or ability that doubles its power resolves. Similarly, an effect that doubles a creature’s toughness gives it +0/+X, where X is that creature’s toughness. Doubling a creature’s power and toughness gives it +X/+Y, where X is its power and Y is its toughness.
January 25, 2019 · Ravnica Allegiance
To double a creature's power, that creature gets +X/+0, where X is that creature's power as the spell or ability that doubles its power resolves. Similarly, an effect that doubles a creature's toughness gives it +0/+X, where X is that creature's toughness. Doubling a creature's power and toughness gives it +X/+Y, where X is its power and Y is its toughness.
April 27, 2018 · Dominaria
To double a creature’s power, that creature gets +X/+0, where X is that creature’s power as the spell or ability that doubles its power resolves. Similarly, an effect that doubles a creature’s toughness gives it +0/+X, where X is that creature’s toughness. Doubling a creature’s power and toughness gives it +X/+Y, where X is its power and Y is its toughness.
September 30, 2016 · Kaladesh
When control of two permanents is exchanged, if those permanents are controlled by different players, each of those players simultaneously gains control of the permanent that was controlled by the other player. If, on the other hand, those permanents are controlled by the same player, the exchange effect does nothing.