There are 13 cards in each suit. The more cards you and partner hold in one suit, the better the trump suit (or “fit”) it becomes. For example, a ten-card fit is fabulous, leaving the opponents with only three cards. It’s rare, though.
In practice, experience shows that a fit of eight or more cards constitutes a decent-enough trump suit. That leaves the opponents with only five cards; five missing cards will normally split 3-2, so three rounds and their trumps will have gone.
The first goal of the bidding is to find a fit. When partner opens or responds in a suit, they guarantee at least four cards. If you have four cards, you have immediately found your fit. Therefore, with four-card support, do not introduce a new suit1.
I was asked this question the other day (relating to a column I wrote in The Times), “Andrew, you suggest, with ♠KJ982, ♥J852, ♦J2, ♣32, to respond 2♥ to partner’s 1♥ opener. I would respond 1♠ – is this wrong?” And the answer (which I expressed as politely as possible) is, “Yes, it is wrong to bid 1♠. You have found your eight-card major fit – hearts. You should support.” Fail to support hearts straight away and partner will never believe you have four-card support. For example, if you bid 1♠, then go back to hearts next time, your partner will think you’re making a (probably) reluctant preference bid, holding three or even only two hearts.
I co-wrote a weighty tome for tournament players over a quarter of a century ago which in truth could have been condensed down to three words, “Support with support"
Exercise: Partner opened 1♥. What would you respond with these?
|Hand a)||Hand b)||Hand c)|
♠ A J
♥ 5 4 3 2
♦ K Q 8 5 2
♣ K 2
♠ 8 6
♥ J 7 4 3 2
♦ A 9 4 2
♣ 3 2
♠ J 8 4 2
♥ A K 2
♦ 8 6 5
♣ Q 5 2
1Unless partner has bid a minor, in which case it makes sense to introduce a major, because game in a major requires fewer tricks.