K–S

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 Roth-Stone system were the two most influential challengers to Standard American bidding in the USA 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 Schenken system. 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:

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