The blackjack in T1 and T2.
A gas arrow in T1 and T2.
A knockout, often referred to as blackjacking, refers to disabling an enemy without killing it. It can be done by using the blackjack (most enemies need to be in an unaware state for a successful knockout) or using Gas Arrows, or Gas Mines. This is a very important mechanism in the Thief Games as hard and expert difficulties typically disallow the player from directly killing anyone and is also a more discreet way of disabling an enemy. When a knockout is performed successfully on an enemy, and does not result in its dead; they will become an Unconscious Body. Some very weak characters, such as Servants, may accidentally be killed from an attack intended to knock them out resulting in a Corpse.
Limitations[]
Certain enemies cannot be knocked out such as magical beings like Fire Elementals and Glyph Statues and creatures that are not alive can also not be knocked out namely the undead and the Builder's Children.
The Mechanist Frogman is fully protected from being knocked out by his Diver's Helmet.
Some normal enemies that can be knocked out carry helmets that protects them from being rendered unconscious by the Blackjack such as the Sheriff Helmet Guards, Mechanist Helmet Guards and Mechanist Frogmans. The helmet guards can still be knocked out with gas based attacks but not the frogman.
Air Mages are immune to gas based attacks and some Mechanists like the Mechanist Frogman and Brother Cavador wears a Diver's Helmet or a protective mask that protects them from gas based attacks.
Blackjacking[]
The blackjack is the primary weapon for knocking out enemies. On most enemies, it can only be used when they are not alerted to Garrett’s presence. Depending on how alert the enemy is, it may also accomplish a knockout if they are searching for Garrett.
In Thief I and II[]
In the first two games, the blackjack works from most angles and simply needs to connect with the opponent while they cannot see Garrett (save the "Surprise" type mentioned below). This includes when they have been blinded by a Flash Bomb or when Garrett is using an Invisibility Potion. Unarmed civilians can be knocked out no matter how aware they are.
Airborne Knockout[]
The Airborne knockout is a 'secret' knockout move. Garrett just has to swing, and then jump. If the blackjack connects and the NPC is knocked out, it will be considered an Airborne Knockout, which will even be tallied in the stats screen after the completion of the mission.
Leaning[]
If a hostile is searching for Garrett, or a hostile can only be taken out from the front, Garrett can lean forward and swing the blackjack (providing he is in a good shadow).
Surprise Blackjack[]
Hostiles have a split second to react to Garrett's presence. In this time, they can still be knocked out, but it requires the player to be very quick. If they have finished their exclamation, it is too late.
In Thief: Deadly Shadows[]
In the third game, the mechanics of the blackjack changed considerably. Enemies and civilians cannot be blackjacked once alerted in any way, and Flash Bombs do not allow a knockout either. Blackjacking is significantly more difficult to accomplish.
The blackjack standing position before attacking
Instead, the blackjack will only work by a proper "sneak attack." Garrett must be behind a completely unaware enemy and the player must wait for Garrett to raise his arm over his head (indicating he is in the right position). Pressing the attack button while Garrett's arm is readied initiates a scripted knockout animation, rendering the opponent unconscious and causing them to drop everything they were carrying, including their weapon.
Gas Based Knockouts[]
Knocking out a Mechanist Patrol (2 characters with one shot).
Gas Arrows and mines emit a type of knockout gas which is effective even on fully alerted enemies and can affect multiple enemies at once if they are close enough to the cloud of gas.