DECK# is an open source annotation protocol that transforms a common deck of French cards (52 cards + 2 Jokers) into a universal substrate for building board games, using only two permanent markers — one black and one red. The basic idea is simple: a deck of cards already brings with it three free informational "aces" (the value from Ace to King, the color red/black, and the suit polarity hearts/diamonds vs. clubs/spades), and DECK# adds on top of that a second layer of geometric symbols — seven glyphs like -, +, △, ○, □ — drawn in the free spaces of the card in standardized positions. These glyphs do not have a fixed meaning: each individual game decides what they represent, exactly like the Piecepack (the system of open components from which DECK# draws inspiration) leaves the designer the freedom to define the rules. The result is a shared vocabulary without imposed semantics: anyone can annotate their deck once, in 15–20 minutes, and then use it to play any title designed on DECK# — a dungeon crawler, a resource management game, an abstract — without purchasing anything proprietary.