When you use a melee weapon or fight unarmed you can use the parry skill to enter a defensive stance. This allows you to block one melee attack at a time and make counter-attacks against less experienced opponents.
Classes: Barbarian, Bard, Cleric, Druid, Fighter, Monk, Paladin, Ranger, Rogue, Swashbuckler; Arcane trickster, Assassin, Blackguard, Divine champion, Duelist, Dwarven defender, Eldritch knight, Frenzied berserker, Invisible blade, Red dragon disciple, Shadow Thief of Amn, Shadowdancer, Warpriest, Weapon master
Requires training: No
Check: The DC of a Parry check is the modified attack roll of the incoming blow. A successful parry means that the attack does not damage the parrying character. Each round, you can parry as many attacks as you can normally make yourself. Additional attacks can be parried, but with a cumulative -3 penalty (so if you have 3 attacks, the 4th parry in one round is -3, the 5th parry is -6, etc).
Special: If the parry is successful and the difference between the roll and the DC is ten points or greater, a counterattack occurs. A counterattack is a free attack made by the parrying character against the parried opponent. The first counterattack in a round is made with a full attack bonus and subsequent counterattacks are made at -5, -10, -15, and so on.
Use: Select the Parry mode. The character will remain in Parry mode until the mode is exited.
For arguments sake and to make it all easy to calculate, take for example two characters:
Parry skill user: ranger 20 (DEX oriented)
Attacker: fighter 20 (STR oriented)
Parry skill roll = 1d20 + 23 (max parry skill at CLVL 20) + DEX mod - 2 (armor check penalty)
Parry DC = incoming blow = 1d20 + 20(BAB of LVL 20 fighter) + 4(weapon) +2 (Weapon focus feats) + STR mod
Assuming that the fighter's STR mod is equivalent to the DEX mod of the ranger and that they both roll exactly the same on the d20, a character with maxed parry will avoid being hit 50+% of the time, which is balanced (Note: Not statistically correct - in the above example the parry adjustments are at -5 with equivalent ability modifiers compared to the incoming blow adjustments and the parry would only be successful 30% of the time; if the parry and incoming blow have the same adjustments the parry will have a 52.5% success rate). The AB of the ranger in this example is not added to the parry check.
If the parry check exceeds the DC by 10 (5 with Improved parry), a riposte attack opportunity is triggered. This does not result in an automatic hit - instead, one attack at the highest BAB is made, which can miss like any other normal attack. Only the first riposte attack is made at max AB; the ones following take cumulative -5 penalties.
If a parry check is failed, it also does not result in an automatic hit. The attacker's blow is then compared to the parrying character's AC normally.
Using a duelist instead of a ranger results in different calculations: at level 7, the duelist will add his level to the Parry skill use. Also, the duelist will not typically have an armor check penalty. This increases the parry roll by 9 points, tipping the balance in the duelist's favor (Note: this increases the chance of successful parry to 70% and a chance of a riposte attack opportunity at 26%; 48% with Improved Parry). Likewise, taking the Skill Focus (Parry) feat will add another 3 points.
With Spellcasting: When casting a spell in parry mode a character will parry attacks, but cannot make a counterattack until the next round. Once a character has begun to parry, any attempt to cast a spell must wait until the next round, meaning that to cast a spell while parrying requires beginning the spell while not under attack; a character under attack will never actually begin casting a spell waiting in their queue. Generally a character will want to change from parry mode to defensive casting mode to throw spells for both safety and convenience.
Parry has limitations due to how the engine handles multiple attacks: it groups all of a character's attacks into three animated attacks "flurries" made at the beginning, middle, and end of the round. If an attacker makes two attacks in the first "flurry", a parry check will not be made against the second attack. This translates to the defender being able to parry three attacks per opponent, yet may still parry more attacks from other opponents. However, due to the timing and duration of animated attacks and parry/ripostes, it is exceptionally rare that a character will achieve up to 6 Parries/Ripostes in a round - they are more likely make only 3 or 4. Although if animations are avoided through actions like movement and attacks of opportunity, the number of parries will not be so limited but are also unlikely to get any Ripostes.
For advice on the use of parry, the reader is encouraged to browse Ragimund's notes on the talk page.
DnD 3.5 comparison
This skill does not exist in DnD 3.5