Dungeons & Dragons game mechanics

In the 3rd edition Dungeons & Dragons role-playing game, game mechanics and die rolls determine much of what happens. These include:


 * ability scores, the most basic statistics of a character, which influence all other statistics
 * special qualities, what innately superhuman characteristics a character has
 * armor class, how well-protected a character is against physical attack
 * hit points, how much punishment a character can take before going unconscious or dying
 * saving throws, a character's defenses against nonphysical or area attacks (like poisons, fireballs, and enchantments)
 * attack rolls and attacks, how effectively a character can score hits against, and inflict damage to, another character
 * skills, how competent a character is in various areas of expertise
 * feats, what special advantages a character has through natural aptitude or training

Ability scores
All characters have six basic statistics:
 * Strength (STR): Strength is a measure of muscle, endurance and stamina combined. Strength affects the ability of characters to lift and carry weights, melee attack rolls, damage rolls (for both melee and ranged weapons,) the Jump, Climb, and Swim skills, several combat actions, and general checks involving moving or breaking stubborn objects.
 * Dexterity (DEX): Dexterity encompasses a number of physical attributes including hand-eye coordination, agility, reflexes, fine motor skills, balance and speed of movement; a high dexterity score indicates superiority in all these attributes. Dexterity affects characters with regard to initiative in combat, ranged attack rolls, Armor Class, Reflex saves, and the Balance, Escape Artist, Hide, Move Silently, Open Lock, Ride, Sleight of Hand, Tumble, and Use Rope skills. It also affects the number of additional attacks of opportunity granted by the Combat Reflexes feat.  Dexterity is the ability most influenced by outside influences (such as armor).
 * Constitution (CON): Constitution is a term which encompasses the character's physique, toughness, health and resistance to disease and poison. The higher a character's Constitution, the more hit points that character will have. Constitution also is important for Fortitude saves, the Concentration skill, and fatigue-based general checks.  Constitution also determines the length of a barbarian's rage. Unlike the other ability scores, which render the character unconscious or immobile when they hit 0, having 0 Constitution is fatal.
 * Intelligence (INT): Intelligence is similar to IQ, but also includes mnemonic ability, reasoning and learning ability outside those measured by the written word. Intelligence dictates the number of languages a character can learn, and it influences the number of spells a preparation-based arcane spellcaster (like a Wizard) may cast per day, and the effectiveness of said spells. It also affects how many skill points a character gains per level, the Appraise, Craft, Decipher Script, Disable Device, Forgery, Knowledge, Search, and Spellcraft skills, and bardic knowledge checks.
 * Wisdom (WIS): Wisdom is a composite term for the characters enlightenment, judgement, wile, willpower and intuitiveness. Wisdom influences the number of spells a divine spellcaster (like clerics, druids, paladins, and rangers) can cast per day, and the effectiveness of said spells.  It also affects Will saving throws, the Heal, Listen, Profession, Sense Motive, Spot, and Survival skills, the effectiveness of the Stunning Fist feat, and a monk's quivering palm attack.
 * Charisma (CHA): Charisma is the measure of the character's combined physical attractiveness, persuasiveness, and personal magnetism. A generally non-beautiful character can have a very high charisma due to strong measures of the other two aspects of charisma. Charisma influences how many spells spontaneous arcane spellcasters (like sorcerers and bards) can cast per day, and the effectiveness of said spells.  It also affects Bluff, Diplomacy, Disguise, Gather Information, Handle Animal, Intimidate, Perform, and Use Magic Device checks, how often and how effectively clerics and paladins can turn undead, the wild empathy of druids and rangers, and a paladin's lay on hands ability.

An ability score is a natural number, with a value of 10 or 11 representing average human ability.

Additional:
 * Comeliness (COM): In the original version of AD&D Comeliness was introduced as a 7th Ability Score/Stat in the supplemental rulebook Unearthed Arcana to differentiate between physical attractiveness and Charisma. Comeliness has not appeared as an officially supported ability score since, although the second edition rules Player's Option: Skills & Powers introduced a subability score named "Appearance."

3.0 and 3.5 editions
Each score has a modifier (mod), where $$Modifier = (Score-10)/2$$, rounded down (drop fractions). So, for example, an ability score of eight ($$(8-10)/2 = -1$$) would result in a modifier of −1, while an ability score of 17 ($$(17-10)/2 = 3.5$$) gives you a modifier of +3. This modifier is added to the appropriate dice rolls. For example, the strength mod would be added to the damage dealt by a sword, the dexterity mod to Armor Class (see below) as the character's ability to dodge attacks, and the charisma mod to an attempt to smooth-talk a merchant.

There are creatures that lack certain ability scores (undead, for example, have no constitution). These are called nonabilities and affect how that creature is treated by certain spells and effects. The aforementioned undead, for example, are immune to almost anything that requires a Fortitude save, unless it can also affect objects. This is not the same as having a score of zero (which causes death, paralysis or unconsciouness depending on the ability in question).

Determining ability scores
There are several methods of determining a character’s initial ability scores during character creation:


 * Rolling dice (3d6): This is the standard method for some pre-3.0 editions. For each ability score, the player rolls 3d6, and adds the values, resulting in scores ranging from three to eighteen, averaging between 10 and 11.
 * Rolling dice (4d6k3): This is the standard method for 3.0 and 3.5 editions. For each ability score, the player rolls 4d6, and adds the three highest values, resulting in scores ranging from three to eighteen, skewed towards higher numbers, averaging 12.24, though the most probable result is 13


 * Predetermined array of scores: Less random, but inflexible.
 * Point buy: In the point buy system, a player has a certain number of points to spend on their ability scores. The more powerful the characters are intended to be, the more points will be available to the players. (Characters are usually more powerful for a more difficult game.) Possible ability scores range from eight to eighteen, and each score has a certain point cost affixed to it, where higher scores tend to cost more points per level than lower ones. This method is used in Dungeons & Dragons Online (and other computerized D&D-based games, such as Neverwinter Nights) to avoid imbalanced characters getting an unfair advantage over other players.

Special qualities

 * Darkvision: This refers to the ability of a creature to see in the dark. It allows the creature to discern shapes (as in normal, daylight vision) but only in shades of grey. Darkvision was introduced in the 3.0 edition of the game to replace both Ultravision and Infravision, which had become seen by some designers of the game as too logically inconsistent to continue using as-is.
 * Ultravision: The predecessor of Darkvision, in pre-3.0 editions of the game, loosely based on the premise of the ultra-violet spectrum, was essentially identical to Darkvision in all but name. This attribute was usually only reserved for those races that lived exclusively in the dark or underground, such as the Drow (Dark Elves).
 * Infravision: Another predecessor of Darkvision, in pre-3.0 editions of the game, which was loosely based on the premise of the infra-red spectrum. It allowed some races to see in darkness by discerning the heat signatures left behind by other creatures. However, under the game rules description it was described as seeing in total darkness just as one would outdoors on a clear night under a bright full moon. This attribute was present in many demi-human races that lived above ground - such as the Elves (non Drow).

Armor Class
Armor Class (AC) is a rating used to determine how difficult it is to damage a creature/character. It is based on several factors such as a creature's natural aversion to physical injury, magical enhancements, and any protective garments worn. The base stat of Dexterity grants bonuses to AC.
 * In some editions of the game prior to 3.0, Armor Class ranges from -10 to 10. Having an AC of 10 was the weakest, and a -10 being the strongest possible written AC.
 * In 3.0 and 3.5 editions, armor class instead starts at 10 and increases. Extremely non-dextrous or non-moving creatures may suffer penalties that lower their armor class below 10.

Hit points
Hit points (HP) are a measure of a characters vitality or health; they are determined by the character's class (certain occupations breed hardier people) or race, and Constitution score. Hit points are reduced whenever a character takes damage. Typically beings fall unconscious at 0 HP. Living creatures reduced to negative HPs continue to lose additional HPs due to bleeding, etc. unless they are stabilized by chance or healing (natural or magical). At -10 HP a being is officially dead.

Saving Throws
Certain situations give characters the chance to avoid special types of danger or attacks. These chances are called saving throws or saves. A saving throw is made when a character would come to harm from extraordinary means such as poisons and magical compulsions in nature.

Pre-d20 System
In the pre-d20 System editions of D&D, there are five categories of saving throws:
 * '''Paralysis, Poison, or Death Magic
 * Petrification or Polymorph
 * Rods, Staves, Wands (magical devices)
 * Spells
 * Breath Weapons (such as with dragons or gorgons)

3rd Edition
There are three kinds of Saving Throws:
 * Fortitude: A Fortitude save represents physical toughness, incorporating stamina, ruggedness, physique, bulk, metabolism, resistance, immunity, and other similar physical qualities. Fortitude saves involve a character's resistance to an effect that directly attacks his health, stamina, or soul. This includes resisting poison, shrugging off the worst of a flesh to stone spell, and ignoring the horrible stench that surrounds a ghast.  Typically, Fortitude saves are the sort of thing that a "tough guy" would be good at.  Fortitude saves are affected by the Constitution base stat.
 * Reflex: A Reflex save represents physical (and sometimes mental) agility, incorporating quickness, nimbleness, hand-eye coordination, overall coordination, speed, and reaction time. Reflex saves involve a character's ability to move out of the way of an incoming object or spell effect as well as his ability to leave an area in a short amount of time. This includes the character's ability to dodge falling rocks and his ability to escape the worst of a Fireball spell.  Typically, Reflex saves are the sort of thing that an agile person would be good at.  Reflex saves are affected by the Dexterity base stat.
 * Will: A Will save represents inner strength, incorporating willpower, mental stability, the power of the mind, levelheadedness, determination, self-confidence, the superego, and resistance to temptation. Will saves involve a character's mental resistance to mental dominance, confusion, stress, and insanity. This includes the character's ability to resist a charm person spell, see through an illusion, and to resist supernatural fear.  Typically, Will saves are the sort of thing that a confident or determined person would be good at.  Will saves are affected by the Wisdom base stat.

Attacking
When a character makes an attack a 20-sided die is rolled to determine success/failure. The result could be adjusted based on any number of possible modifiers the character or its intended target have.

The number added to the die roll is actually several different modifiers combined, coming from different places. These modifiers include the character's proficiency with the specific weapon and weapons in general, the quality of the weapon (masterwork craftsmanship or magical enhancements), the modifier of the ability associated with the weapon (strength for melee, or close-quarters, weapons, and dexterity for ranged weapons), magical effects improving/hampering the character's ability to attack, and any special experience the character has fighting a certain foe.


 * In the 1st edition of the game, the final result is compared to a table along with the target's Armor class to see if the attack hits. Every general class type had its own matrix-style table, while "monsters" or "creatures" pre se used the same as the generic fighter character type.
 * In the 2nd edition of the game, if the final result equals or exceeds the attacker's THAC0 (the pre-recorded number the character needs To Hit Armor Class 0"), the attacker has successfully hit a target with armor class 0. If the target has an armor class different from zero (which is far more likely than not), the target's Armor Class is subtracted from the attacker's THAC0, and that number is what the attacker's roll must equal or exceed to see if the attack hits.
 * In 3.0 and 3.5 editions, the attack hits simply if the final result is equal to or greater than the target's Armor Class.

Actions
The combat mechanic is turn-based and operates in rounds. A round is a discrete time interval (approximately 6 seconds, game-time) in which all involved parties act in the combat. The order in which parties involved in the combat act is determined by Initiative.


 * In pre-3.0 editions of the game, characters are allowed to move their speed and attack every round, or perform a reasonable combination of other actions.
 * In 3.0 and 3.5 editions, what a character can and cannot do in a given round is more codified; a character may perform one full-round action, one standard and one move action, or two move actions in a round, along with any number of free actions, and a single swift or immediate action. Unlike other types of actions, immediate actions may also be taken during someone else's turn, though that counts as using the immediate action slot for the character's following turn.

Skills
Dungeons and Dragons, starting with the 2nd Edition of the game and continuing to the current 3rd Edition, has many skills that characters may train in. In the 2nd Edition these were broken down into Weapon and Non-Weapon Proficiencies. In the 3rd they are all simply referred to as "Skills", not to be confused with "Feats", below. Characters gain skill points for buying skill ranks based on class, level, and intelligence. Some skills can only be taken by certain classes, such as Read Lips or Animal Empathy. These skills are called exclusive skills. Others can be used even if the character has no ranks in that skill (i.e., is not trained in that skill).

A skill check is always a d20 roll, with bonuses from the number of skill ranks, the skill's key ability, and any miscellaneous modifiers (from spells or racial abilities, for instance). Sometimes, a skill check may be aided by favorable circumstances (such as you brandishing a weapon while using Intimidate) or hampered by unfavorable circumstances (such as using improvised tools to pick a lock).

An example of a skill is Search, which is Intelligence-based; an example of a miscellaneous modifier which could be applied to search is the +5 competence bonus for a character wearing the "Goggles of Minute Seeing". Other skills include Diplomacy (CHA), Escape Artist (DEX), Swim (STR), various Knowledge skills (like Knowledge (Arcana) or Knowledge (Local)) (INT), Spot (WIS), and Concentration (CON).

A "check" is successful when the roll is higher than or equal to the difficulty class (DC) of the task. Usually, the Dungeon Master sets the DC. Sometimes the DC is set by the result of something else's check, this is an "opposed check". An example of an opposed check is spot against hide: the character is trying to see something else that is hidden/trying not to be seen.

Feats
Feats were introduced in 3rd edition of Dungeons & Dragons. They are similar to the proficiencies of 2nd edition, giving characters more depth in a structured way for the game.

A feat is an advantage, often some special option for the character (such as a special combat maneuver) or some modification to game options and the mechanics involved. Feats can be contrasted with skills, which were also introduced in the same edition, in that using a feat does not usually require the particular success/fail roll that skills do. Instead of possessing a certain rank at a skill, a character either possesses a feat or does not.

There are many different types of feats. Some are magical; such as "Silent Spell" which allows a spellcaster to cast a spell without speaking words. Others are combat-related, such as "Weapon Focus" which gives a bonus to attack rolls when a character is using a certain weapon (such as a Longsword). Another example is "Leadership", which allows the character to attract henchmen and followers.

Many feats, especially the most powerful ones, require certain prerequisites (such as related feats or minimum ability scores) in order to select that feat. Some feats provide continual effects, while others, typically ones that have some cost to use or can be used to a variable degree, must be declared before use.

Feat types: Ambush, Bardic, Epic, Exalted, Divine, General, Heritage, Item Creation, Metamagic, Metapsionic, Psionic, Tactical, Wild, Vile