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

• Open in the minor, of course, then try to show the major on Round 2, but not at the cost of reversing with a minimum hand.

For example, with:

KJ96
2
K2
AJ7642

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:

8
A1086
QJ10765
AJ

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

• With a minimum hand, tend to repeat the 6-card major.

For example, rebid 2 after 1-1 with: 6 AQ10765 K765 K2.

• With extras, show a decent 4-card minor on Round-2, hoping to get a 3rd bid. On the 3rd bid, repeat the major and partner will expect a 6-4 with extra values.
  
  So, rebid 2 after 1-1NT with:

  K108654
  A2
  2
  AKJ2. Hopefully, partner takes a second call, and you can bid spades next.

• With 6 spades and 4 hearts, after 1-1NT, it is a good idea to "always" (even with a minimum) mention the hearts. There is a good chance of a 4 game (since responder can have lots of hearts for his 1NT response to 1).
  So, after 1-1NT, rebid 2 with:

  AQ8765
  KQ65
  43
  2.

Notes:

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:

J76542
5
AKQ10
K2. But, with:

KQJ1076
A2
Q765
3,

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.