Threat range

Each weapon has a threat range for critical hits.

A critical threat is obtained when you roll a d20 to hit your opponent and the result (before applying any bonus or penalty) is within the threat range of your weapon (even if your modified attack roll wouldn't beat the opponent's AC).

A critical threat means you have a chance to score a critical hit. To do so, a confirmation roll is made, with the same to-hit bonus being applied (plus extra confirmation bonuses such as Seeker, Kensei enhancements, Fighter Critical Accuracy, etc.); in this roll, 1's do not automatically fail to hit. This roll must beat the target's AC in order to "confirm" the critical hit.

In DDO, a confirmation roll can still succeed on a roll of 1; it does not auto-miss on such a roll.

If that roll also results in a hit against the target's AC, the critical hit is confirmed, and the base damage is rolled once for every critical hit multiplier. If the confirmation roll is a miss, then the attack is not a critical hit and is rolled as a normal attack (IE, auto-success on natural 20 or test vs. AC for non-20).

All weapons create a critical threat on a roll of 20, but some have a larger threat range, extending down to 19-20 or even 18-20.

Ways to increase threat range

 * The Keen (slashing/piercing) or Impact (bludgeoning) weapon prefixes double the weapon's base threat range. A Kukri (base threat range 18-20) with Keen, for example, will have a threat range of 15-20. This does not stack with Improved Critical.
 * The Improved Critical feat doubles the base threat range for any equipped weapon in a specific category (slashing/piercing/bludgeoning/ranged/thrown). This does not stack with Keen/Impact.

Competence bonus
Competence bonus to threat range can come from various sources. Bonuses to critical threat range usually are not doubled Improved Critical. Multiple effects granting competence bonus don't stack.
 * Artificer's Reconstructed Armaments: +1 to chosen weapon type
 * Barbarian's Critical Rage: +2 threat
 * Barbarian's Occult Metalline: +1 threat, +1 multiplier
 * Bard's Swashbuckling enhancement modifies the threat / multiplier as follows
 * Dagger, Shortsword, Throwing Dagger: +1 Threat, +1 Multiplier
 * Light Mace, Light Hammer, Kama, Sickle, Dart, Shuriken, Throwing Axe, Throwing Hammer: +2 Threat, +1 Multiplier
 * Kukri, Rapier: +1 Multiplier
 * Handaxe: +2 Threat
 * Light Pick: +1 Threat Unlike most bonuses, Swashbuckling bonus to threat is doubled by Improved Critical.
 * Fighter's Keen Edge enhancement adds 1 to the threat range of a his/her selected weapon group.
 * Gnome's and Deep Gnome's Weapon Training adds +1 for light hammers, light maces, light picks, shortswords, shortbows, light crossbows, and light repeating crossbows.
 * Holy Sword: +1 to threat and multiplier for all weapons, but not shields. Available as:
 * Paladin 4th level spell
 * Cleric War Domain power - SLA
 * Staff Specialization, available for Rogues and Monks, adds +1 to threat and multiplier for quarterstaves
 * Knife Specialization, available for Assassins, adds +1 to threat for daggers. Additionally, it adds +1 multiplier for daggers and kukris. Unlike most bonuses, Assassin's bonus to threat is doubled by Improved Critical.
 * Master Thrower (Halfling): +1 threat for thrown weapons
 * Deadly Striker: while centered: +2 for Kama/Shuriken, +0 Handwraps/Quarterstaff, +1 with any weapon
 * Empty Hand Mastery: +1 threat and multiplier for Handwraps
 * Advanced Sneak Attack, available for Rangers, adds +1 to threat with all weapons (not shields/unarmed).
 * Vistani: +1 threat and multiplier for daggers and throwing daggers

Insight bonus

 * Bard's Exploit Weakness enhancement. Every time you don't crit, you get a +1.
 * Legendary Dreadnought's Pulverizer adds +1 threat for bludgeoning weapons. Find details and exceptions in the LD article.

Morale bonus
Knight's Training feat:
 * +1 for Battle Axes, Heavy Maces, Morningstars, War Hammers
 * +2 for Battle Axes if you have Improved Critical Slashing

Swords to Plowshares feat:
 * +1 for Quarterstaves, Kamas, and Sickles

Untyped / unclear bonus type

 * Blessing of Silvanus: +2 for Mauls
 * Divine Crusader's Celestial Champion provides +1 critical threat range with all weapons. (Does not apply to handwraps or while in an animal form.)
 * Violence Begets Violence - While Centered, when an opponent misses you in melee, you gain +1 critical threat range for 6 seconds. This stacks up to 5 times and is reset if you critically hit.
 * Vanguard's 5th core ability increases the threat range and multiplier of your shield by 1.

Many active attacks, often coming from enhancements, feats, or epic destiny abilities, modify the critical threat and/or multiplier of a single attack or short duration. These effects usually stack with everything else. For example:
 * The Paladin's exalted smite
 * Legendary Dreadnought's Cleave-like attacks
 * Fury of the Wild's Adrenaline
 * Vistani Vendetta

Weapon damage comparison
Note that this calculation does not include additional damage from strength modifier, weapon enhancement bonus, weapon prefix/suffix effects, etc., resulting in x2 multiplier weapons looking better than they really are. This calculation also assumes a 100% critical confirmation roll, in this case resulting in lower threat weapons (which usually carry a better multiplier) looking better. To understand the breaking point, besides the obvious critical effects, consider how often you hit an opponent. The harder it is to hit (attack roll vs AC), the better it is to have a higher threat range weapon as you always hit on a non-confirmed critical.

STR modifier example: (on hit with vanilla weapons, does not consider glancing blows, feats or enhancements)
 * a strength 10 (+0) fighter
 * Bastard Sword: 5.5 base expected, 11 on crit → 6.1 avg
 * Khopesh: 4.5 base expected, 13.5 on crit → 5.4 avg
 * a strength 30 (+10) fighter
 * Bastard Sword: 15.5 base expected, 31 on crit → 17.05 avg
 * Khopesh: 14.5 base expected, 43.5 on crit → 17.4 avg
 * a strength 40 (+15) fighter
 * Bastard Sword: 20.5 base expected, 41 on crit → 22.55 avg
 * Khopesh: 19.5 base expected, 58.5 on crit → 23.4 avg