Watermark

A watermark is a faint imprint made into the text box area of certain Magic cards. In Contraptions, they are placed in the bottom right-hand corner.

History
The first card to bear a watermark was Incoming! from Unglued.

Events
Prerelease cards bear their expansion symbol as a watermark since Onslaught's, similarly release cards bear their set symbol or logo. Other event cards not specific to a set bear the logo of that event, such as Friday Night Magic promo cards.

Affiliations
Some cards in the Ravnica block were given watermarks showing the guild from which they came. The Ravnican watermarks were used again in the later Ravnica sets.

In the Scars of Mirrodin block every single card that was not a basic land or a planeswalker bore the watermark of one of the two warring factions: Phyrexians and Mirrans.

Khans of Tarkir uses watermarks for the different clans, with Fate Reforged adding watermarks for the different dragons' broods.

Strixhaven: School of Mages uses watermarks for the different colleges within the school.

Watermarks were also used for the factions of Unstable.

Affiliation watermarks are searchable in Gatherer and Scryfall.

Mechanics
Foretell from Kaldheim was the first mechanic with an associated watermark (a raven).

Watermark matters
Unstable has a small "watermark matters" theme with Hammerfest Boomtacular, Stamp of Approval and Watermarket. In this set, the faction marks found in the bottom right corner of Contraptions also count as watermarks.