Threshold

Threshold is an ability word that gives a card different or additional characteristics if its controller has seven or more cards in their graveyard. It was introduced in Odyssey.

Description
Threshold indicates that an additional or different ability is available if the controller of the card with threshold has seven or more cards in their graveyard. It is written in one of the following ways:
 * Threshold — As long as there are seven or more cards in your graveyard, [static ability], [self-replacement], [activated ability] and/or [triggered ability]
 * Threshold — [static ability], [self-replacement], [activated ability] and/or [triggered ability]. Activate this ability only if seven or more cards are in your graveyard

History
Threshold appeared as a keyword ability on 84 cards in the Odyssey block. However, rules-wise it wound up being a bizarrely complex characteristic-setting ability. “Threshold — [text]” meant “As long as you have seven or more cards in your graveyard, [this object] has ‘[text].’” [Text] could be anything. . . and if you didn’t have seven or more cards in your graveyard, the game pretended that [text] didn’t even exist. Threshold was seen in all colors. However, in Odyssey and Torment, blue was notoriously good at putting cards into the graveyard, and therefore had fewer threshold cards (it would have had an unfair advantage over the other colors).

It was later realized that Threshold actually was a precursor to ability words that were formally introduced with Saviors of Kamigawa. Ability words are so simple that they don’t actually have any rules meaning. The formal change was made with the release of Time Spiral. The “threshold” section from the rules was eliminated, and all threshold cards received errata.

Deep-Sea Terror from Magic Origins and Search for Azcanta in Ixalan essentially feature the mechanic, but without the ability word. Jace, Vryn's Prodigy and River Serpent have a five-card version.

The mechanic made an reappearance in Modern Horizons on a reprint (Nimble Mongoose and a new card (Excavating Anurid).

Related mechanics
Some creatures in Odyssey with threshold are lycanthropes, meaning they change from one thing into another; the creature type line captures the before and after, but the art only shows one, usually after.

Several related "threshold mechanics" like metalcraft also allow a card to swap between various states based on some external criteria somewhere in the game.

Jace's Phantasm from Magic 2015 was the first card to have reverse threshold (caring about the number of cards in the opponents graveyard, albeit 10 instead of 7).

Rules
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Rulings

 * Instants and sorceries with threshold check if seven or more cards are in your graveyard when they resolve, not when they're cast or put on the stack.
 * Activated and triggered abilities that permanents have at threshold check the number of cards in your graveyard only when the abilities are activated or triggered. Even if you have fewer than seven cards in your graveyard when the ability resolves, it will still resolve normally.
 * Static threshold abilities are simply "on" when you have seven or more cards in your graveyard and "off" when you don't.