The Combat Round (PHB)

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If an encounter escalates into a combat situation, the time scale of the game automatically goes to rounds (also called melee rounds or combat rounds). Rounds are used to measure the actions of characters in combat (or other intensive actions in which time is important).
A round is approximately one minute long. Ten combat rounds equal a turn (or, put another way, a turn equals 10 minutes of game time). This is particularly important to remember for spells that last for turns, rather than rounds.
But these are just approximations—precise time measurements are impossible to make in combat. An action that might be ridiculously easy under normal circumstances could become an undertaking of truly heroic scale when attempted in the middle of a furious, chaotic battle.
Imagine the simple act of imbibing a healing potion. First, a character decides to drink the potion before retiring for the night. All he has to do is get it out of his backpack, uncork it, and drink the contents. No problem.
Now imagine the same thing in the middle of a fight. The potion is safely stowed in the character's backpack. First, he takes stock of the situation to see if anyone else can get the potion out for him, but, not surprisingly, everyone is rather busy. So, sword in one hand, he shrugs one strap of the pack off his shoulder. Then, just as two orcs leap toward him, the other strap threatens to slip down, entangling his sword arm. Already the loose strap keeps him from fully using his shield.
Holding the shield as best as possible in front of him, he scrambles backward to avoid the monsters' first wild swings. He gets pushed back a few more feet when a companion shoulders past to block their advance. His companion bought him a little time, so he kneels, lays down his sword, and slips the backpack all the way off. Hearing a wild cry, he instinctively swings his shield up just in time to ward off a glancing blow.
Rummaging through the pack, he finally finds the potion, pulls it out, and, huddling behind his shield, works the cork free. Just then there is a flash of flame all around him—a fireball! He grits his teeth against the heat, shock, and pain and tries to remember not to crush or spill the potion vial. Biting back the pain of the flames, he is relieved to see the potion is still intact.
Quickly, he gulps it down, reclaims his sword, kicks his backpack out of the way, and runs back up to the front line. In game terms, the character withdrew, was missed by one attacker, made a successful saving throw vs. spell (from the fireball), drank a potion, and was ready for combat the next round.
==What You Can Do in One Round==
Whatever the precise length of a combat round, a character can accomplish only one basic action in that round, be it making an attack, casting a spell, drinking a potion, or tending to a fallen comrade. The basic action, however, may involve several lesser actions.
When making an attack, a character is likely to close with his opponent, circle for an opening, feint here, jab there, block a thrust, leap back, and perhaps finally make a telling blow. A spellcaster may fumble for his components, dodge an attacker, mentally review the steps of the spell, intone the spell, and then move to safety when it is all done. It has already been shown what drinking a potion might entail. All of these things might happen in a bit less than a minute or a bit more, but the standard is one minute and one action to the round.
Some examples of the actions a character can accomplish include the following:
* Make an attack (make attack rolls up to the maximum number allowed the character class at a given level)
* Cast one spell (if the casting time is one round or less)
* Drink a potion
* Light a torch
* Use a magical item
* Move to the limit of his movement rate
* Attempt to open a stuck or secret door
* Bind a character's wounds
* Search a body
* Hammer in a spike
* Recover a dropped weapon
There are also actions that take a negligible amount of time, things the character does without affecting his ability to perform a more important task. Examples of these include the following:
* Shout warnings, brief instructions, or demands for surrender, but not conversations where a reply is expected.
* Change weapons by dropping one and drawing another.
* Drop excess equipment, such as backpacks, lanterns, or torches.