Reverse Bids are not strictly speaking a Convention; rather they are a logical partnership understanding.

Compare these two sequences:
2♣ 2

In the first sequence North has bid his two suits in the "normal" way, higher-ranking suit first. He could hold the same number of cards in each suit and might have a minimum strength opening hand. Notice that if South preferred the first suit he could say so at the 2-level with a 2 bid.

In the second sequence North has bid the suits in "reverse" order, lower-ranking suit first. He must have more ♣s than s or he wouldn't bid them this way. But now notice that if South preferred the first suit he would have to go to the 3-level to say so with a 3♣ bid.

When a player opens 1 of a suit, then bids 2 of a higher-ranking suit at his second turn, the second bid is called a Reverse. He should have a stronger than minimum hand, 17 or more points, and must have more cards in the first suit than in the second.
A Reverse is forcing on Responder for one more round.

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