It's one of the most-asked questions:
"When I'm 6-4 and open the 6-card suit, should I repeat the 6-card suit or bid the 4-card suit next?"
Bad news -- there is no "right answer." There is no definitive rule or agreement among teachers and experts.
Here are some mainstream thoughts/guidelines:
With 6 of a minor and 4 of a major
For example, with:
Open 1 and rebid 1 if partner answers in a red suit. But, if partner responds 1NT, you aren't strong enough for 2 (which shows a good enough hand to be at the 3-level when partner likely goes back to clubs). With:
open 1 and rebid 2 after a 1 (or 1NT) response. You are not strong enough to reverse.
With 6 of a major and 4 of another suit
For example, rebid 2 after 1-1 with: 6 AQ10765 K765 K2.
AKJ2. Hopefully, partner takes a second call, and you can bid spades next.
These are guidelines only; there is still room for judgment. Some players are strong believers that with a 6-card major and a 4-card side suit they should always bid the 4-card suit on Round 2. They feel it is important to show at least 9 cards in their hand (partner will know they are at least 5-4 in 2 suits) as opposed to only 6 cards (which is all partner knows when you repeat the 6-card suit on Round 2).
"Form of scoring" is certainly relevant. At matchpoints, where playing a higher-scoring partial is so important, the scales should tip more towards repeating a 6-card major (to avoid the risk of ending in a lower-scoring minor).
"Suit quality" is also a big consideration. Even though it is a minimum hand, I'd rebid 2 after 1-1NT with:
K2. But, with:
I'd surely rebid 2 and eschew the diamonds.
"2/1 Auctions." If the auction begins, for example, 1-2, especially if it is GF, there will be plenty of time to repeat a 6-card suit later (there will be a "later."). So, when in a GF, it is usually best to show the 4-card suit ASAP.
What about 6-5? Click here to read more.