Unlike all other kinds of damage, combat damage is dealt only by creatures and only during the combat damage step of the Combat Phase. This type of damage does not use the stack and is equal to the power of the creature dealing the combat damage.
All creatures in combat deal combat damage at the same time, except when one or more creatures have first strike or double strike. In this case there are two damage assignment steps. In the first, all creatures in combat with first strike and double strike deal combat damage at the same time. Then, in the second, all creatures in combat that have not yet dealt combat damage or have Double Strike deal combat damage (this means creatures that lose Double Strike after dealing combat damage do not deal damage again).
An attacking creature that is blocked assigns combat damage among the creatures that block it. Blocked creatures with the Trample ability may also assign combat damage to players in certain cases.
An attacking creature that is not blocked assigns combat damage to the defending player.
A blocking creature assigns combat damage among the attacking creatures it blocks, which is one creature except in the case of certain abilities.
Attacking creatures that are blocked but have the blocking creature removed from combat before the Damage Assignment Step deal no combat damage. Attacking creatures with Trample may deal combat damage to the defending player in this case.