Metalcraft

Metalcraft is an ability word introduced in Scars of Mirrodin, and was subsequently also featured in the rest of the Scars of Mirrodin block. It is exclusive to the Mirran faction.

Description
Metalcraft gives a bonus to a permanent or a spell if its controller also controls three or more artifacts.

Most cards with Metalcraft are boosted with increases to power and/or toughness and/or keyword abilities. Artifact cards with Metalcraft abilities are counted as one of the three required artifacts for the ability activation. Therefore, Chrome Steed becomes a 4/4 if you have two other artifacts in play.

Metalcraft also appeared in Commander 2018 and Modern Horizons 2.

Rulings

 * Some metalcraft abilities that appear on instants and sorceries use the word "instead". These spells have an upgraded effect if you control three or more artifacts at the time they resolve. For these, you only get the upgraded effect, not both effects.
 * Metalcraft abilities of instants and sorceries that don't use the word "instead" will provide an additional effect if you control three or more artifacts at the time they resolve.
 * Some metalcraft abilities are activated abilities of permanents. To activate such an ability, you must control three or more artifacts at the time of the activation. The ability will resolve as normal even if you no longer control three or more artifacts as it resolves.
 * Some metalcraft abilities are static abilities. These abilities constantly check whether or not you control three or more artifacts to see if the bonus applies. With one exception ("Stoic Rebuttal"), these abilities function only on the battlefield.
 * A creature with a static metalcraft ability that increases its toughness, such as Auriok Sunchaser, loses that bonus as soon as its controller controls fewer than three artifacts. If damage marked on that creature is greater than or equal to its toughness after the metalcraft ability stops applying, the creature is destroyed and put into its owner's graveyard as a state-based action.
 * Triggered metalcraft abilities include an intervening "if" clause. You must control three or more artifacts at the appropriate time in order for these abilities to trigger, otherwise they do nothing. The game will check again as these abilities resolve. If you don't control three or more artifacts at that time, these abilities will do nothing.
 * Some metalcraft abilities appear on artifacts. Since you already control that artifact, you need to control only two others for the ability to work.

Notable Metalcraft cards

 * Rusted Relic - Is a non-creature artifact that becomes a creature when you control three or more artifacts.
 * Stoic Rebuttal - This is the only card that benefits from a reduction in casting cost due to Metalcraft.
 * Argent Sphinx and Kuldotha Phoenix - These cards have additional mana costs to activate their Metalcraft abilities.
 * Mox Opal - A Mox-type card which may only tap for mana when you control three or more artifacts.
 * Vedalken Certarch</c> - Has a tap to activate Metalcraft ability, Mox opal</c> also technically has a tap to activate Metalcraft ability.

Trivia

 * Metalcraft was featured as rules card 2 of 5 in the Scars of Mirrodin set and as rules card 2 of 5 in the Mirrodin Besieged set.
 * Kaladesh featured Toolcraft Exemplar</c> and Inventor's Fair</c>, which used the Metalcraft mechanic, without using the keyword. Tezzeret, Artifice Master</c> also shares this quirk, whose design integrated the storyline of the Kaladesh arc.