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![]()
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