Players who do not trust each other blindly will usually agree on common rules, defining which kind and number of proxies, if any, is allowed for each player. These rules may vary drastically depending on the participants and the occasion. However, some restrictions are implied naturally by common sense and plain physics.
Indistinguishable on the back The main issue to guarantee fair play in a card game is that all cards in the deck must be indistinguishable for any player who does not view the front side (if
card sleeves are used, the term 'card' means the sleeve with the card inside). Ideally, all cards should be
indistinguishable in the following characteristics to effectively prevent
cheating. • Card size and shape, including the typical rounding cut on the edges. • The card's total
weight, its
center of gravity and, ideally, the
moment of inertia (which implies a homogeneous distribution of mass on the surface). • Overall and local
stiffness and elasticity - all cards should behave equally on bending. • Overall and local thickness. • Feel and relief (
tactile characteristics) of the card, especially elevations and cavities on the surface on both sides. • The image printed on the back side, including its shininess. • Overall and local transparency, when examined with a light from behind. Besides these physical implications, it should be considered that someone (the players or a judge) will need to control the validity of the cards - which may prove difficult with some of the above points. Therefore, the use of proxies is sometimes further constrained to only one method of fabrication, for instance. The difficulties of control can also be an argument to totally prohibit the use of proxy cards.
Unambiguous mappings on the front Once the front of a proxy is revealed to the other players, it must be clear to everyone what it is meant to substitute. The decisions of what a player's proxies are meant to substitute must be made before starting play. If two proxies are meant to substitute different cards, they must be easily
distinguishable by looking at their front side. Ideally, the label of a proxy should be enough to tell what it's meant to substitute. Alternatively, a legend or agreement can be used to prohibit players from changing these mappings during play. Another issue for the front side labeling is to maintain a fluid game play. Poor labeling will likely cause unpleasant disruptions, even slips and mistakes caused by accidental confusion. It is therefore desirable that each proxy is labeled with the name of the card it substitutes, and its basic game-relevant characteristics, and erase all decorations and printed information that may be misleading. Any relevant information that is not written on the card should at least be found in the legend.
Agreements and coded rules Additional rules can restrict the number of proxy cards allowed in a deck, and the physical ways in which they may be created. Such rules can be a simple agreement between two players, or they may be defined by the host of a tournament.
Magic: The Gathering tournaments sanctioned by the
DCI allow the use of proxy cards only to replace cards damaged during play (e.g. water is spilled on a deck mid-tournament, causing some cards to be marked). Some third party organisations hosting
Magic tournaments permit participants to include a set number of proxy cards in their decks (5 and 10 being common amounts), a clause that especially comes up for the cost-intensive
vintage format. ==Common ways of fabrication==