after the concept of vulnerability was introduced to contract bridge in 1926 and the adoption of boards for duplicate bridge, a set consists of 16 (or 32) boards, owing to the 16 possible deal combinations arising from the four states of vulnerability for each of the four dealer positions. By the mid-1930s, thirty or forty different types were being sold. The first oblong boards were made of paper by William McKenney in 1928; the first metal boards were by F. Dudley Courtney in 1931. The most common modern boards are rectangular in shape and made of plastic replacing earlier models made of paper covered pasteboard, sheet metal, wood and more recently aluminum. Boards designed as foldable wallets, originally made of leather, primarily used in England, are now generally made of pliable plastic. While the concept of the contract bridge duplicate board evolved from the duplicate whist tray, Set of boards A set of boards for duplicate bridge typically contains 32 boards and sometimes as many as 36. The actual number of boards used in a particular session depends on the type of tournament, the number of tables, and the choice of movement used. Often some of the higher numbered boards (e.g. 25 to 36) are not needed. The dealer and vulnerability markings for each board number are standardized in the laws of the game, utilizing all the possible permutations. The dealer is rotated clockwise in successively numbered boards. Four combinations of vulnerabilities also change, but they are also shifted circularly. Thus, a set of 16 boards has the following markings: The scheme is repeated in the subsequent set of 16 (i.e. on boards numbered 17–32); boards 33-36 repeat the scheme of boards 1–4. Preparation of the deal At the beginning of a session, the boards are prepared in one of several ways: Predealt – the sponsoring agency has prearranged the cards in the boards, and the boards are given to the players "ready for play". Player shuffle – before the start of play, each table receives a number of boards each containing 13 cards in each of its four pockets. The cards are removed from the pockets of a board by the players, shuffled together and dealt into four equal piles. Each pile is placed into a pocket of the same board. Each board is prepared in the same manner and stacked in numerical order ready for play. The players