Walsh is not really a convention, but more of a "Style."
It deals with the responses to a 1
opening bid -- and the follow-ups.
Responses to 1
Let's start by examining the auction 1—Pass—1
. Should responder bid up the line (bidding 1
with 4 or more) or should he bypass diamonds? For example, what should he respond to 1 with each of these hands?
A Q 8 2 
3 2 A Q J 5 4 8 7- A Q 8 2 3 2 J 9 8 5 4 8 7
- A Q 8 2 3 2 J 5 4 2 8 7 2
With 1), respond 1 and later show spades.
With 2) and 3), bypass the diamonds and show the major right away—responder might get only one chance to bid. This is the modern treatment (some call it "Walsh.")
The theory is that with a "one-bid" hand you bypass diamonds so you can make sure to introduce your 4-card major into the auction. If you know you will get to bid twice, you can bid your diamonds first (even with 4-4 in diamonds and the major). "Knowing you can bid twice," means responder has at least opening-bid strength.
When using this style, after the partnership starts 1
-1, the opener should rebid 1NT with any balanced hand (even with a 4-card major). He can do so safely with the knowledge that a 4-4 major-suit fit won't be missed. For example, opener has:
K J 9 3 K 7 4 7 3 A J 5 3.
He opens 1 and hears partner respond 1. He should rebid 1NT. He knows there is no chance of missing a 4-4 spade fit. (If there is one, then partner has enough to bid again and will bid spades next). The 1 responder can't hold hand 2) or 3) above. Note: if you are not using the "Walsh" (bypass ) style, then opener would need to rebid 1 with this hand.
Notice that when using Walsh style, opener's rebid in this auction: 1-1-1MAJ, promises real clubs. Opener won't have a balanced weak notrump (or he'd have rebid 1NT). He will be unbalanced when he opens 1 and then bids 1-of-a-major.
Example auctions:
Opener:
QJ5
KJ54
Q2
A1076
Responder:
K104
86
K10743
J54
| Opener |
Responder |
| 1
|
1
|
| 1NT |
Pass |
There is no need for opener to mention the hearts. Responder either doesn't have 4 (as here), or is strong enough that he will introduce them on the next round of bidding.
Opener:
QJ5
KJ54
Q2
A1076
Responder:
4
AQ102
K10743
KQ4
| Opener |
Responder |
| 1
|
1
|
| 1NT |
2
|
| 3
|
4
|
| Pass |
|
Responder's 2 shows diamonds and hearts and is forcing.
Opener:
5
KJ54
QJ2
AQ1076
Responder:
742
97
K1074
KJ42
| Opener |
Responder |
| 1
|
1
|
| 1
|
2
|
| Pass |
|
Opener's 1 is not only natural (4 cards), but promises an unbalanced hand (if balanced, he would have opened or rebid in notrump). Responder should go to the known club fit instead of bidding 1NT.
Alertable?
ACBL does not require alerts for the bids in this article (since they are natural). On the 2022 ACBL convention card, you can check the box which says "Bypass 5+" next to the 1 response to 1. It is in black, which means not alertable.
