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- The Kaplan–Sheinwold (or "K-S") bidding system was developed and popularized by Edgar Kaplan and Alfred Sheinwold during their partnership, which flourished during the 1950s and 1960s. K-S is one of many natural systems. The system was definitively described in their 1958 book How to Play Winning Bridge and later revised and retitled to The Kaplan-Sheinwold System of Winning Bridge in 1963. Kaplan–Sheinwold and the system were the two most influential challengers to Standard American bidding in the US in the 1950s, 1960s, and 1970s. Although K-S is not frequently played in its original form in the 21st century, many of its features (though not the 12-14 point 1NT opening) survive in the popular 2/1 Game Forcing system. Additionally, a few elements of Kaplan–Sheinwold (notably Five-Card Majors) have become accepted as part of Standard American practice. Among modern experts, Chip Martel and Lew Stansby play a system closely modeled on K-S, with loads of gadgets. In the late 1960s, the Precision Club system grafted a strong, forcing opening of 1♣ onto K-S, in effect following earlier suggestions by Marshall Miles that five-card majors and the weak no trump be added to the . Kaplan viewed Precision with distaste, noting the disadvantages, both theoretical and at-the-table, of combining a strong club with five-card majors. The principal features of K-S, as revised in the 1960s, are these: 1.
* Weak no trump. An opening bid of 1NT promises 12–14 high card points (HCP). Transfers are not used, and Stayman is non-forcing. Kaplan's highly successful partnership with Norman Kay used "Timid K-S," which departed from the original K-S structure by using a strong no trump when vulnerable. 2.
* Five-card majors, with limit raises. A 1NT response is forcing and responder's double is negative. 3NT is the strong, forcing raise. Two of a minor over a major suit opening is game forcing, unless rebid. 2♥ over 1♠ can be weaker (minimum is 10 points and a five card suit) than two of a minor, so as not to miss a good heart partial. Kaplan preferred to open 1♠ with 5-5 in the black suits and a minimum hand. 3.
* Minor suit openings are strong or unbalanced, or both, because the weak no trump handles all weak, balanced hands. A 1NT rebid by opener shows a strong no trump (15–17 HCP) and a 2NT rebid shows 18–20 HCP. Opener's reverses are forcing. Opener's simple rebids (e.g., 1 m – 1M; 2m) are restricted to absolute minimum hands, and tend to show six cards in the minor. Opener's jump rebids (e.g., 1 m – 1M; 3m) are enormously strong, promising a hand just shy of a forcing opening bid. After a 1♦ opening, a rebid of 2♣ shows the strength and pattern of a reverse, and opener's jump to 3♣ shows a weak hand with 5-5 in the minors. 4.
* In response to one of a minor, responder shows a four card major if possible with a weak or moderate hand. But with values for game, responder first bids a longer side suit, even the other minor, and may rebid in a major. For example, the sequence 1 ♣ - 1 ♦; 1♥ - 1♠ may show a strong hand with long diamonds and four spades, but it may also be a "moderately strong hand without a spade stopper." [1] 5.
* Inverted minor suit raises are used (a single raise is strong, a double raise is weak and preemptive). 6.
* Weak two bids, including 2♦. 7.
* 2♣ is the only strong, forcing opening. 8.
* Defensively, simple overcalls are taken to have the same range as an opening bid, and take-out doubles emphasize distribution. (en)
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