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Weapon skill represents your proficiency with a particular weapon. The basic formula for your maximum weapon skill at a particular level is (5 * level). The rate at which you gain them depends on your weapon speed. It is typically better to use a fast weapon to train up your skills. Instant attacks like Sinister Strike and Hamstring help to increase your weapon skill faster. Note that attacking Evading enemies will not increase your weapon skill.
It has been confirmed that intellect increases the rate at which weapon skills are learned. Before Patch 2.0, this was listed in the tooltip but was removed shortly after that. If you want to level a weapon skill, try asking for an intellect buff or using a scroll which can help the process.
In Patch 2.3, stats on melee weapons which granted an increase to weapon skill were replaced by an increase to expertise rating. Items which granted ranged weapon skill increases instead now grant increases to hit or critical strike rating.
Effects of Weapon Skill
NOTE: With respect to the impact of Weapon Skill on miss rates and +hit, both the supplied preBC information and the Crezax post are no longer accurate. For the correct information, please see the updated "Chance to Miss" section below.
For each point that your weapon skill exceeds your opponent's defense, you gain the following:
Note: These numbers are from pre2.0, and may or may not still be correct. Crezax on the EU forums has made a post contradicting these values (see below).
 Your chance to miss decreases by 0.04%.
 Your chance to score a critical strike increases by 0.04%.
 Your opponent's chance to block your attack decreases by 0.04%.
 Your opponent's chance to parry your attack decreases by 0.04%.
 Your opponent's chance to dodge your attack decreases by 0.04%.
CM Crezax made a post[1] on the EU forums outlining the effect of weapon skill on higherlevel mobs. Note that this refers only to monsters 3 levels higher than the player in PVE (i.e. boss level mobs). Corresponding numbers for 1 and 2 levels higher than the player are not known, but are likely lower.
Let's say you increase your weapon skill by 4. Against a mob 3 levels higher than you, you get: 0.8% lower chance to miss, 0.4% lower chance to get dodged, 2.4% to be parried, and a 0.8% higher chance to crit.
Note: These numbers given by the CM are also questionable and should be taken with a grain of salt. It is difficult at this point to determine which numbers are correct without more playercollected data.
Effectively, you can consider weapon skill to be the inverse of the defense skill.
Additionally, the Patch 2.0.1 notes said:
 The player will gain 0.1% to their critical strike rating per weapon skill against monsters above their level.
Presumably this is only for Weapon Skill over your level's natural cap (Level * 5). It was not known if you also got the additional 0.04% crit chance per additional +skill.
Weapon skills are maxed in pvp combat. While this means leveling a weapon for pvp is unimportant, racial attribrutes, talents, and weapons with +skill still affect pvp hit/crit rating. Many players, however, see the +skill in PvP as unimportant and you will still see many Orcs with PvP reward weapons besides the Axes.
Glancing Blows
Against mobs with defense roughly equal to or higher than your Attack Rating (i.e. your weapon skill), you will have a chance to land a Glancing blow instead of a normal hit. A glancing blow only occurs on white melee damage, cannot crit, and does less damage than a normal blow. If the attacker's weapon skill is less than his level * 5, the damage reduction will be proportional to the difference between the attacker's weapon skill and the target's defense.
Back before patch 2.3, when Weapon Skill could be raised above your level * 5 from gear or racial bonuses, doing so never affected the chances of landing a glancing blow, although the damage reduction from glancing blows was affected until patch 2.0 or so.
Chance to Miss
It has been definitively proven that, at least with respect to calculating miss rates and chance to hit, both the preBC formula and the Crezax post are inaccurate.
 If the difference between the mob's Defense Skill and your Weapon Skill is less than or equal to 10 then the formula for calculating your base miss chance against that mob is: 5% + (Defense Skill  Weapon Skill)*.1%
 If the difference between the mob's Defense Skill and your Weapon Skill is greater than 10, then the formula for calculating your base miss chance against that mob is: 7% + (Defense Skill  Weapon Skill  10)*.4%
Note that these 2 formulae only govern attacks made against mobs. With attacks against players (either by mobs or by other players) where the attacker's Weapon Skill is lower than the target's Defense, the base Miss, Dodge, Parry, and Block chances are much less; see the Defense article for details.
Applying these formulae gives the following base miss rate for a Level 80 character with a 400 Weapon Skill:
 v. Level 70 mob: 5.0% / dualwield: 24%
 v. Level 71 mob: 5.5% / dualwield: 24.5%
 v. Level 72 mob: 6.0% / dualwield: 25%
 v. Level 73 mob: 9.0% / dualwield: 28%
 v. 32.79 hit rating reduces your chance to miss by 1%
Thus if you are a Level 80 character with a Weapon Skill of 400, you need a Hit Rating of 295 (9.00%) to almost never miss a shot against a Level 83 boss (or skull mob) with a special attack or singlewield autoattack. What this means is that there is a big +hit benefit to keeping your Weapon Skill within 10 levels of the mobs you are trying to fight. For example, by improving your Weapon Skill from 390 to 395, you effectively reduce your chance to miss against Level 81 mobs by 3%! However, after getting to 395, increasing your Weapon Skill the next 5 levels, to 400, only reduces your chance to miss by an additional 0.5% (or 3.5% in total). Also, note that there is a huge jump in miss rate reduction by going from 394 to 395 Weapon Skill. This is the point where you switch from one formula to the other, and so this particular single point of Weapon Skill is worth a dramatic +1.4% hit against a Level 83 mob/boss.
Chance to be Dodged, Parried, and Blocked
The formulas for determining Dodge, Parry, and Block rates for attacks made vs. mobs are different from the formula above for determining Miss rate and are not all definitively known. As of Patch 2.4, the best working models seem to be the following:
 The base chance for your attack to be dodged by a mob is: 5% + (Defense Skill  Weapon Skill)*.1%
 The base chance for your attack to be parried by a mob:
 If the difference between the mob's Defense Skill and your Weapon Skill is less than or equal to 10, your base parry rate is: 5% + (Defense Skill  Weapon Skill)*.1%.
 If the difference between the mob's Defense Skill and your Weapon Skill is greater than 10, your base parry rate will be much higher. Exact figures are not known, but base parry rates of Boss mobs (effectively level 73 when attacked by level 70 players) are estimated to be between 1015%, and may vary from one Boss to the next.
 The base chance for your attack to be blocked by a mob is probably the same as the base dodge rate, i.e.: 5% + (Defense Skill  Weapon Skill)*.1%
Crit Cap
Whitedamage attacks made by a player with fullytrained weapon skill against a mob 3 levels higher have a fixed 24% chance of being glancing blows.
Therefore, with no +skill/+hit gear on, attacks made by a level 80 dual wielder against a level 83 mob have the following table from the front:
Result  Chance 

Miss  28.0% 
Dodge  6.5% 
Parry  At least 10% 
Glancing Blow  24.0% 
Block  6.5% 
Combined hit + crit  At most 24% 
Attacks against mobs made from the back cannot be blocked or parried (thus, attacking from behind will never trigger "parry haste" on the mob). Losing those values from the chart makes the "combined hit + crit" entry appropriately larger. Players cannot dodge, parry, or block attacks from the back.
It is not as bad if the same character attacks the same mob from the front with a twohanded weapon:
Result  Chance 

Miss  9% 
Dodge  6.5% 
Parry  At least 10% 
Glancing Blow  24.0% 
Block  6.5% 
Combined hit + crit  At most 43% 
This is a 43% + your +hit crit cap.
All these crit caps could have been increased by increasing your Expertise (0.5% increase per point from the front) and by +hit (1% increase per percantage increase in your Hit chance).
Dual wield full raid buffs example from the front:
+0 hit +50% crit: 6.5% dodge, 10% parry, 28.0% miss, 6.5% block, 25% glancing blow, 24% crit = 26% of your crit rate did nothing +6% hit +40% crit: 6.5% dodge, 10% parry, 6.5% block, 22.0% miss, 25% glancing blow, 30% crit = 10% of your crit rate did nothing +15% hit +31% crit: 6.5% dodge, 10% parry, 6.5% block, 13% miss, 25% glancing blow, 31% crit, 8% hit = no +crit was wasted in this example
Dual wield full raid buffs example from the back:
+0 hit +50% crit: 6.5% dodge, 28.0% miss, 25% glancing blow, 40.5% crit = 9.5% of your crit rate did nothing +6% hit +40% crit: 6.5% dodge, 22.0% miss, 25% glancing blow, 40% crit, 6.5% hit = no +crit was wasted in this example +15% hit +33% crit: 6.5% dodge, 13% miss, 25% glancing blow, 33.0% crit, 22.5% hit = no +crit was wasted in this example
DPS Implications
Hit Rating is very important due to the crit cap. The following shows the difference in white damage between various stat options:
Assuming 30% crit and +0% hit from the front, its 6.5% block for say 80% damage (depends on a lot of things) 24% crit for 200% damage (impale doesn't work on whitedamage attacks!) 25% glancing blow for 70% damage 44.5% miss, dodge, or parry for no damage per swing its an average of .065 * .8 + .24 *2 + .25 * .7 = 70.7% of your damage per swing
Assuming you add +5% hit, the numbers become... 6.5% block for say 80% damage 29% crit for 200% damage 25% glancing blow for 70% damage 39.5% miss, dodge, or parry no damage so per swing its an average of .065 * .8 + .29 *2 + .25 * .7 = 80.7% of your damage per swing
Increasing your weapon skill (at level 70+)
For ranged DPS (bows/guns)  Dr. Boom is an excellent choice  just bring enough ammo, and attack at will. Hunters have the best options with him due to autoshoot. This no longer works as of 3.2.
Another good place to increase ranged DPS for higher level characters is Plague Sprayer in Zul'Drak. They are level 80 Elites that fly above Kolramas. They have a high amount of health and the only attack they have is a plague beam that shoots straight down.
The Primordial Drake Eggs in Sholazar Basin also work well for this; they are high 70's, and do not attack back.
An option to raise weapon skills are the Gordok Spirits in Dire Maul North. They are located in the last room. Make sure that you DON'T kill the boss since the spirits would despawn after his death. The spirits don't fight back and have an unlimited amount of health, so it's the optimal place to just go AFK while automatically weapon skill raising progresses. Note that the game won't log you out since you will be infight as long as you hang out there increasing your skills. ***As of 3.1 this no longer works***
Increasing your weapon skill beyond (level * 5)
Weapon skill ratings have been replaced by Expertise.
Itemization
Itemization for +weapon skill is spotty.
2.0.1 Skill Rating conversion
The Weapon Skill figures in the above item chart used to be from the old pre2.0 system, where +weapon skill was given to items directly. Under the new combat rating system, the above items now all have a Weapon Skill Rating bonus instead.
It takes 2.5 points of Weapon Skill Rating at level 60, and 3.9 points of Weapon Skill Rating at level 70, to increase your actual Weapon Skill by 1.
In applying the miss rate formulae listed in the "Chance to Miss" section above, fractional Weapon Skill levels are dropped, not rounded. So, say you have are a Level 70 character, and you equip a piece of gear that gives +15 Mace Skill Rating. In theory, this equates to 3.8 +Weapon Skill Level. And, in theory, Blizzard could have used the fractional .8 in its subsequent calculations of hit and miss, or, perhaps, rounded it to 4.0. But it doesn’t work that way. Instead, the fractional .8 Weapon Skill is simply dropped altogether. You get 353 Weapon Skill for purposes of all hit and miss calculations.
Another interesting point is that if you are able to stack your Weapon Skill above 365, you will find that each additional point of Weapon Skill above 365 continues to decrease your chance to miss by .1% per Weapon Skill level. So if your Weapon Skill is 366, your chance to miss is 4.9%, if 367, 4.8%, etc.
Talents
 Weapon Expertise talent increases skill with Swords, Fists, and Daggers by 5 (rank 1) or by 10 (rank 2).
 Mace Specialization talent increases skill with Maces by 2 per talent point spent (max 10).
 Weapon Mastery talent increases skill with all weapons by 2 (rank 1) or by 4 (rank 2).
 Combat Expertise talent increases skill with all weapons by 2 per talent point spent (max 10).
Racial Bonuses
 Humans: +5 sword skill, +5 mace skill, +5 twohanded sword skill, +5 twohanded mace skill
 Dwarves: +5 gun skill
 Orcs: +5 axe skill, +5 twohanded axe skill
 Trolls: +5 bow skill, +5 throwing weapon skill
See also
External links
 2.0.1 Patch Notes
 Zoroaster's Crit, Hit, Weapon Skill and damage formulas
 ElitistJerks' Roguecraft 101 (contains mention of base Dodge chance of bosses)