Compare these two sequences:
| NORTH | SOUTH | NORTH | SOUTH | |
| 1♦ | 1♠ | and | 1♣ | 1♠ |
| 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.
Lesson available.