Your partner has opened the biding. When does your response in a major guarantee at least a 5-card suit? The answer is in this article--where there will often be interference (an overcall or double).
Without an opposing overcall, it is fairly easy. Here are the rules:
A response on the 1-level in a major guarantees only 4 (of course, it could be longer).
Examples:
1
-P-1
1-P-1
1
-P-1
1-P-1
1-P-1*
*If using Flannery, normally this would show 5+spades
A response of a major on the 2-level (other than jumps) can occur only in this one auction:
1-P-2
That should guarantee at least 5 hearts.
Any other direct major-suit responses are jumps, so clearly not 4-card suits.
If there is a takeout double after each opening bid above, the responses have exactly the same meaning with regard to major-suit length.
What if there is an overcall? It is necessary to have an understanding of the rules for negative doubles. That is the basis for some of the answers in the chart below:
| Opening | Overcall | Response | # of cards promised |
| 1 |
1 |
1 MAJ | 4+ (double = 4+ & 4+) |
| 1 |
1 |
1 |
5+ (double = exactly 4) |
| 1 |
1 |
2 |
5+ |
| 1 |
1 |
1 |
5+ (double = exactly 4) |
| 1 |
1 |
2 |
5+ |
| 1 |
2 minor | 2 |
5+ |
| 1 |
2 minor | 2 |
5+ |
| 1NT | Overcall | Bid of a Major | 5+ |
After a 1 overcall, a major-suit response can be made with only 4. But, after a 1 overcall, a 1 response guarantees at least 5. Two-level responses guarantee at least 5.
Summary of the summary: If they overcall anything other than 1, then responder's bid of a major guarantees 5 or more. If they overcall 1, then responder is allowed to bid 1 or 1 with only 4.
Warning: This needs to be studied, studied again and memorized. It isn't complicated, but causes complication if it isn't digested properly.
Coming soon: Article on what a 1 or 1 responder does on the next round if he really has 5 (or more).