Brown sticker

Brown sticker is a category of contract bridge conventional agreements defined by the World Bridge Federation (WBF).

Brown sticker conventions are considered to be difficult to defend against, and thus are only permitted at the highest levels of tournament play in most locations. Only highly unusual methods (HUMs) have a higher classification.

The official definition can be found on the WBF website [ [http://www.worldbridge.org/departments/systems/policy.asp WBF System policy] ] .

A short summary would be that a convention qualifies for a brown sticker if it fulfills any of the following criteria:
# An opening bid of 2♣ through 3♠ may be weak and does not promise a known suit.
# An overcall of a natural opening on the first level does not promise 4 cards in a known suit.
# A weak two-suited bid where one of the suits by definition may be only 3 cards or shorter.
# It is used for protection of psychic bids, or systemically required psyches.

Notable exceptions are the Multi 2 diamonds (due to its popularity), a natural 1NT overcall, and cue bids on strong hands to force partner.

The bottom line is, that if a bid is weak (or potentially weak having multiple options) and does not promise 4 cards in a defined suit (when weak) then it is likely to have a brown sticker.

References