From bridgebum.com
See also: Gavin Wolpert's webinar "Minor Suit Opening Structure"
See also: Larry Cohen's article "Walsh Style"
Playing traditional 5-card majors, a 1♣ opening bid asks for natural responses "up the line". Responder must bid ♦ with 4+ diamonds before showing a 4-card major. Simple, right?
This approach was commonplace until the 1970s, when American expert Richard Walsh popularized his Western Scientific bidding system. With less than game-forcing strength, Walsh espoused bypassing the ♦ response to show a 4-card major immediately.
When to Use Walsh
With less than 12 points
Holding 4+ diamonds and a 4-card major, responder should bid the major first. It's important to show the major first and foremost, because responder might not have the strength to bid again.
East: ♠ K Q 8 7 ♥ 10 8 3 ♦ A 9 7 6 3 ♣ 7
| West | North | East | South |
| 1♣ | pass | ? |
Bid 1♠.
East: ♠ K Q 8 7 ♥ 10 8 3 2 ♦ A 9 7 6 ♣ 7
| West | North | East | South |
| 1♣ | pass | ? |
Bid 1♥. When holding 4-4 majors, show the hearts first. You can still locate a spade fit if partner rebids 1♠.
How “extreme” to take the Walsh philosophy is a matter of partnership agreement. For instance, some pairs might bypass 6 strong diamonds to bid 4 weak spades… and some might not.
East: ♠ 7 6 4 2 ♥ 10 7 ♦ A K Q 8 3 ♣ 7
| West | North | East | South |
| 1♣ | pass | ? |
Do you really want to bid 1♠? Many if not most Walsh players would consider a descriptive 1♦ instead.
With 12 or more points
With game-forcing strength, responder can afford to bid his diamond suit first, and rebid his major next.
East: ♠ K 5 2 ♥ A K 8 3 ♦ K J 10 7 3 ♣ 7
| West | North | East | South |
| 1♣ | pass | ? |
Bid 1♦. You are willing to reverse and show your hearts later.
Benefits of Walsh Responses
Why play Walsh? First, you can find a major suit fit faster. This can be important when the opponents compete.
| ♠ 8 2 ♥ A 10 6 4 ♦ A 4 ♣ A Q J 5 2 |
♠ K 10 3 ♥ 8 5 2 ♦ 6 5 2 ♣ K 10 9 4  ♠ A J 9 7 5 4 ♥ J 3 ♦ Q 8 7 ♣ 8 3 |
♠ Q 6 ♥ K Q 9 7 ♦ K J 10 9 3 ♣ 7 6 |
| West | North | East | South |
| 1♣ | pass | 1♥ | 2♠ |
| 3♥ | 3♠ | 4♥ | pass |
| pass | pass |
East bids 1♥ holding fewer than 12 points. This helps West identify the heart fit after South's preempt. Imagine what would happen if East initially responded 1♦. West would have a tough time introducing hearts at the 3-level.
Another ironic benefit of Walsh is the occasional chance of jamming the opponents yourself. Check out this deal:
| ♠ Q J 6 ♥ K 8 2 ♦ 7 4 ♣ A K 10 8 5 |
♠ 4 3 2 ♥ J 7 6 4 ♦ A 6 5 ♣ Q J 9 ♠ K 10 8 ♥ A Q 10 9 ♦ K 10 8 2 ♣ 4 3 |
♠ A 9 7 5 ♥ 5 3 ♦ Q J 9 3 ♣ 7 6 2 |
| West | North | East | South |
| 1♣ | pass | 1♠ | pass |
| 1NT | pass | pass | pass |
If East bids 1♦ first, South has a safe 1♥ overcall (or double). Instead, the 1♠ Walsh response makes it much riskier for South to compete at the 2-level.
Note that in ACBL games, Walsh responses are not alertable. However, the “Frequently bypass 4+♦” checkbox should be marked under the Minor Opening section.
Opener's rebids
When the auction begins 1♣-1♦ opener is normally expected to rebid 1♥ or 1♠ with 4 cards. Playing Walsh, however, opener may choose to rebid 1NT with a 4-card major and a balanced hand. There are a couple reasons for this.
First - in his fine book “Modern Constructive Bidding”, Marshall Miles advocates against opening 1♣ with a 3-card suit and rebidding a 4-card major. The risk is that partner might take a preference to clubs with only a 3- or 4-card holding.
Second, responder is guaranteed to bid again with a 4-card major and game-forcing values. Therefore, it's more economical and preemptive for opener to bid 1NT directly with balanced shape
Examples:
| ♠ 10 7 3 ♥ A 8 6 4 ♦ A K ♣ Q J 8 6 |
♠ A 5 4 ♥ K 3 ♦ 10 9 4 3 ♣ A 7 5 4 ♠ Q J 6 2 ♥ Q J 10 5 ♦ 8 6 ♣ 10 9 2 |
♠ K 9 8 ♥ 9 7 2 ♦ Q J 7 5 2 ♣ K 3 |
| West | North | East | South |
| 1♣ | pass | 1♦ | pass |
| 1NT | pass | pass | pass |