I agree with the previous comments (and feel more strongly about it than others, and would likely have failed to be as polite).
You should think of 2
♣ as a necessary evil in a standard system. It is a self-preempting nebulous bid, starting the hand description at the top of the 2-level or sometimes even at the 3-level. If your partnership has the (in my opinion inferior) agreement to include certain weak hands in the 2
♣ opening the pressure is greater still. Auctions after 2
♣ are complicated and crude, involving guesswork and praying that you didn't miss a slam or didn't end up in the wrong strain. Despite all that, sometimes we want to open it anyway because the following two conditions simultaneously hold:
- If we open at the 1-level, there is a significant chance of being passed out.
- Despite this, we might well make game in the scenario where everybody passes.
I think it is common to misinterpret these two conditions as "if I can give partner a 5-count with a side singleton and 3-card support and be a favourite to take 10 tricks in my major, I must open 2
♣". Of course partner never holds that hand, and when partner does the opponents have a fit and strength and would have bid. I think a more relevant condition is whether you can distribute the remaining strength over the three unseen hands in a way that nobody would enter the auction. Roughly this means that if you have a long suit (e.g. 6(+) cards), it is almost always safe to open at the 1-level. Someone will most likely strain to get their suit in, and then you get to bid again. I've opened 1
♣ on a 25-count and 1
♦ on a 24-count before (in Dutch Doubleton), though this was extreme.
Further driving home the point, the 2
♣ doesn't say anything about distribution. If you have a very shapely hand it is critical to describe your shape to partner along with your strength. With these 'high playing strength but low hcp' powerhouses you should expect the auction to be
competitive. If you open 2
♣ you may find yourself on the wrong end of a 4
♥ bid in no time at all - obviously you bid 4
♠, but how is poor partner to know when to pass and when to bid on? Use the bidding space you have to show your shape by opening at the 1-level, and then show the strength later by bidding again.
My rule of thumb is that I want 22 hcp for a 2
♣ opening. I have opened 2
♣ before on certain 21-counts, of course, but only on a low fraction. It's not meant as another 'rule of X' though - it's just that opening at the 1-level is almost always better, especially with some shape. When in doubt, with hands in the 19-21 range, I would strain to open 1-of-a-suit. I do have the advantage of playing a system that caters to these hands slightly better than standard, but I take the same action in standard systems. Usually, if the cards really are distributed in a way that all three other players pass, game wasn't making anyway (we might have 21-opposite-5 or so but there's no communication between the hands, but the much more likely scenario is 21-opposite-3 misfitting as the opponents didn't overcall and everything is a bust). I don't understand or support the fearmongering of a perfect pass opposite, especially in the modern bidding landscape of aggressive overcalls and straining to respond - though even without those opening 1-of-a-suit is simply offering you favourable odds compared to 2
♣.