A screen is a device used in some tournaments in duplicate bridge that visually separates partners at the table from each other, in order to reduce the exchange of unauthorized information and prevent some forms of cheating. It is a panel made of plywood, spanned canvas or similar material, which is placed vertically, diagonally across the playing table, with a small door in the center and a slit beneath it. The door is closed during the bidding stage, and the players place their calls using bidding cards on a movable tray, which slides under the door. After the opening lead, the door is opened, but its size allows the players only to see the hands and cards played from the opposite side of the screen, not their partner's face.
Screens are normally used on high-level competitions, such as World Bridge Olympiads, national teams championships and similar. They are always accompanied with bidding boxes and a tray for moving the bids across. Screens were first introduced in Bermuda Bowl competition in 1975, at the home venue in Bermuda; however, they didn't prevent the foot-tapping scandal involving two Italian players. Following that event, screens used in high-level events extend under the table to the floor forming a barrier running diagonally between two table legs.
Rationale
Laws of Duplicate Contract Bridge state that "Players are authorized to base their calls and plays on information from legal calls and plays and from mannerisms of opponents. To base a call or play on other extraneous information may be an infraction of law.". Also, "After a player makes available to his partner extraneous information that may suggest a call or play, as by means of a remark, a question, a reply to a question, or by unmistakable hesitation, unwonted speed, special emphasis, tone, gesture, movement, mannerism or the like, the partner may not choose from among logical alternative actions one that could demonstrably have been suggested over another by the extraneous information." In other words, if a player receives such an unauthorized information from the partner (hesitation being the most frequent source), he may not act according to its (perceived) consequences (provided it is not an obvious thing to do, but "obvious" has rather high standard according to the laws).
The most frequent means for transmitting the unauthorized information are: Hesitation (break in tempo) during bidding and play. Players are supposed to make their bids and plays in a tempo as even as possible; an experienced player will often make a slight pause even when there is no problem at all, in order to maintain the same tempo if a problem occurs. However, when a player has a bidding or play problem, his hesitation often tells much about his holding. For example, a long contemplation over a double of opponents' contract indicates uncertainty whether the contract would be beaten, and "invites" the partner to bid on ("pull the double") without defensive values; a hesitation which card to play when the declarer plays towards dummy's king suggests possession of an ace, etc. Questions asked by the partner to the opponents. The players are entitled to know the meanings of opponents' bids, and they may ask them whenever it's their turn to bid or play. However, if a player has a habit to skip the question with a worthless hand and pass, and asks with some values (and passes if he doesn't like opponents' reply), his partner can draw conclusions about his holding. Questions asked by the opponents to the partner. If opponents ask the partner about the meaning of a bid or play, his answer may reveal a previous misunderstanding in the partnership. Also, the partner's alert of a bid he shouldn't have alerted, and vice versa, may indicate that someone has forgotten the agreement. Partner's gestures and facial expressions can be telling, even if inadvertent.
Outright cheating by transmitting information by gestures, finger play, pencil play and similar methods agreed in advance is strictly prohibited, and violators can be banned from duplicate play for several years or forever.
Screens are supposed to remove all issues except changes in tempo altogether: partners don't see each other, and since the screenmates are required to communicate only by writing, the partner cannot hear the (mis)explanation. Also, each player alerts both his own and his partner's bids and explains the meaning only to his screenmate, so even if a misunderstanding occurs, the partners will not become aware of it.