[twcho]

I use to open 1♦ for hands with 4♦ and 5♣ (not reversible strength) but I am fully aware that many hold different idea and will open 1♣.

What is the pros and cons for these two different openenings?

[Hanoi5]

I prefer to open 1♣. I call this idea of opening in Diamonds so as not to reverse afterwards 'Italian'. It probably comes from the times 'canapé' was in vogue.

It usually means that responder should prefer to pass the 2♣ rebid than risk a preference in a possibly shorter suit. As I abhor this idea I prefer to open 1♣ and then:

- support partner's Major with 3 cards
- Bid NT with a stopper in the unbid Major
- Rebid clubs

Three possible lies instead of one. But it's better in my opinion.

[SteveMoe]

I think the right perspective is to consider your possible rebids when partner makes an inconvenient response.

Holding ♠AQ2 ♥2 ♦K432 ♣K5432, a 1♥ response might seem inconvenient but many would continue with 1♠ comfortably...
With ♠2 ♥AQ2 ♦K432 ♣K5432, a 1♠ response is cumbersome but 1N rebid by opener offers only modest distortion.
It's the 2=2=4=5 hands where opening 1♦ helps by preparing a rebid when the major doubletons are weak. With Kx Kx in the majors a 1NT rebid is not a distortion. With xx Kx you have to hope for a ♠ reply or risk 1N with xx there (ugly).
...all to avoid rebidding a 5-card minor, particularly when Hxxxx or similar.
So the answer is a matter of context, not rule.

It's fairly easy to play that 1♦-1X-2♣ by opener shows 4=5 or 5=4.

[mr1303]

Playing standard methods, it depends a lot on the relative strengths of the 2 suits. AKQx K5432 I would happily open 1D and rebid 2C. Also agree with a lot of what Steve says.

[mr1303]

Playing standard methods, it depends a lot on the relative strengths of the 2 suits. AKQx K5432 I would happily open 1D and rebid 2C. Also agree with a lot of what Steve says.

[awm]

I prefer 1♣. My plan is to raise if partner bids a major where I have three, or to bid 1NT otherwise. Some reasons for this approach:

(1) Opening 1♦ and rebidding 2♣ gets us to the wrong minor if partner has equal length. This can happen even if partner has invitational values with 3-3 in the minors (1♦-1♠-2♣-3♦? or do you expect partner to bid 2NT regardless of heart holding?)
(2) Opening 1♦ and rebidding 2♣ can get us to 2♦ when partner has two diamonds and three clubs. It is normal to make a "false preference" here on something like 8-9 hcp to keep the bidding open in case opener has just short of a game force. Obviously this leads to a violation of Burn's Law (playing in a suit contract with less than half the trumps).
(3) Rebidding 1NT allows us to find many more good partials, such as a 5-3 heart fit when partner is 5/5 in the majors, or a 4-3 heart fit with a ruffing value in the three-card hand (same auction with partner 5/4 majors), or 1NT itself when that's the best spot (which it sometimes is).
(4) If I end up opening 1♣ and then raising partner's major, he will get a better picture of my hand for game/slam purposes than if I open 1♦ and raise partner's major. Of course, I could open 1♦ and then rebid 2♣ despite having 3-card support, but this leads to many ridiculous results (the major is almost invariably better, plus getting us to some good games that we miss by not showing our support+side singleton).
(5) If the opponents end up bidding a lot, opening my longer suit is more likely to get the right lead from partner or to help partner decide whether to sacrifice or whether to double.

Opening 1♦ helps in the specific situation where we have a 4-4 diamond fit that we cannot find after partner bids 1♣-1M. This is certainly a possible situation, but frequency-wise it can't possibly outweigh all of the above.

I've played a bunch of strong club, where you basically have to open 1♦ on these hands (because 1♣ is unavailable), and I find that to be one of the big negatives of the system! I certainly wouldn't volunteer to bid this way in a natural system!

[Fluffy]

I totally disagree with SteveMoe
with 2245 I will always rebid 1NT, anything else is a losing strategy IMO
with (13)45 you can show both minors, rebid 1NT with a singleton, rebid clubs or show 4 in a major where you only have 3.

For people who don't like 1NT rebid nor showing 4 where there is 3, they have 2 reasonable choices, rebid clubs or bid 1♦ and then 2♣.

What I do is to look at suit quality, rebidding ♣Jxxxx is not a good idea, but rebidding ♣KQ9xx is fine for me. Diamonds quality also plays a role obviously.

[Zelandakh]

I think that players who have experimented with an artificial club system are generally more likely to open 1♦ with these shapes, not because they necessarily like it but just because they are more likely to consider the option. In general, my preference is to open 1♣ in a natural system (read Acol) unless the clubs are very poor and treating the hand as having primary diamonds seems like a better description. There is also an additional solution here that has not been mentioned. If you play a 1♦ opening as always unbalanced, one possibility for a rebid structure is to play 1NT as showing clubs at least as long as diamonds and 2♣ as showing diamonds longer than clubs. In this style, if you want to treat a hand as balanced then you have to open it 1♣ regardless of minor suit lengths.

Whether to treat 2245 hands as balanced (always) is possibly a thread all of its own. Suffice to say that this can lead to problems when partner has both majors, in addition to missing some minor suit fits. I certainly have an option for showing a 2245 hand after a 1♦ opening (strong club, weak NT system) and would feel a little straight-jacketed if that was completely unavailable, especially for the 15-17 hands.

[Codo]

If you play an old system- where partner will bid his four card diamond suit- or a new system- where he will transfer with no major and diamonds with 1 ♠, I would allways open 1 ♣.

Within most actual systems, I have no strong feelings. I play it with one partner, but up to now I still do not see the imps rolling in just because of this treatment.

[y66]

Here's how some other experts bid 1-3-4-5 hands that aren't good enough to reverse.

[RSClyde]

I'll always open 1♣
If I'm 2245 I'll rebid 1nt. If (13), if partner bids my 3 card major I'll raise, and my singleton I'll bid 1nt.
I actually made a video on the topic:
Rebidding 1nt with a singleton