The term inverted minors refers to a treatment used by the Kaplan-Sheinwold (K-S) bidding system for the popular card game bridge (Precision, as originally structured, also used inverted minors over a 1♦ opening). Under this treatment, a single raise of opener's minor suit is strong (usually at least 9 HCP up to 20 HCP), and a double raise is pre-emptive, showing a maximum of 8 HCP. Thus, the treatment inverts the usual structure of the single raise as weak and the double raise as showing limit raise strength.
The single or double raise promises at least five card support for the minor. In the original version of K-S, and as further developed by Kaplan through the 1990s, both raises also deny a four card major.
This treatment is well suited to a bidding system that employs the weak no trump. With 1NT used for balanced hands in the 12-14 HCP range, responder to a 1♣ or 1♦ opening knows that partner has either a good hand, 15-17 balanced, or an unbalanced hand with genuine length in minor suit. If opener has a three-card minor, he most often has a strong notrump hand; then, if desirable, it's easy to veer into no trump following the single raise.
In either case, the partnership is on solid ground after a single raise: opener is either unbalanced or strong, and responder has at least an average hand. The single raise leaves plenty of room to explore for the best contract, and even leaves room for an incautious opponent to enter the bidding and be penalized heavily.
After a double raise, the partnership seldom bids on. The idea, of course, is to pre-empt the opponents and cause them to miss their best contract. The exception occurs when opener has a very strong hand, perhaps one with which he intended to jump in notrump at his second turn (showing 18-20 HCP in K-S). That hand might simply bid 3NT after the double raise, but the decision can be a difficult one: it's opener, not the opponents, who has been pre-empted. Many players, holding a 19 HCP balanced hand, would continue to 3NT over a double raise only when vulnerable at IMPs.
This typical (but minimum) single raise in clubs is taken from the system book: ♠ 765 ♥ K84 ♦ A5 ♣ Q9652.
This is minimum for a double raise: ♠ 765 ♥ 84 ♦85 ♣J97652. One might use it at favorable vulnerability only.
- The Kaplan-Sheinwold System of Winning Bridge, Fleet Press Corporation, 1963.