If you respond 2 to 1, you are guaranteeing at least 5 hearts.
That is the only major-suit response (other than jumps) that guarantees at least 5 cards. (For a review of this subject, click here.)
Responses on the 1-level guarantee only 4. When your response to partner's opening bid is 1 or 1, you are showing 4+ cards.
What if you actually have 5 or more? How do you convey that information?
It depends on opener's rebid. Let's take a look:
|1 any||1 or 1|
If the opener's second bid is a suit (either repeating his own suit or bidding a new suit), responder should not repeat his major with only 5. He needs 6 or more cards to repeat his suit. So, in each auction below, responder should have 6 or more of his suit:
Responder shows 6 or more hearts (not forcing/weakish).
Responder shows 6 or more spades (invitational).
I know what you are thinking. What if responder has 5 of his major and opener happens to have 3? Too bad. Bidding isn't perfect. Sometimes opener will actually raise with 3. For example, I would raise 1 to 2 if partner responded 1 after I had opened 1 with:
What if opener's rebid is 1NT or 2NT? For example:
|1 any||1 or 1|
|1NT or 2NT||??|
This slightly changes things. Now, opener has at least a doubleton in every suit (a notrump rebid shows a balanced hand). If responder repeats a 5-card major, it will land him in no worse than a 5-2 fit. Not perfect, but contrast this to rebidding a 5-card major in the auctions above. In those situations, it could lead to a 5-1 or even 5-0 fit.
So, should responder repeat his major with only 5 cards after hearing a notrump rebid? As much as I like to give Yes/No (100%) answers, I can't do it here. I firmly espouse always showing a 5-card major when partner has opened 1NT. When the rebid is 1NT, I tend to rebid the 5-card major (hoping for an 8-card fit, but relying on at worst a 7-card fit). But, not always. For one, some partners occasionally rebid 1NT with a singleton in my suit. I don't advocate that style, but others do. If I fear a 5-1 fit, repeating my major (unless it is a very good 5) is scary. Also, if I have scattered values and a so-so 5-card suit, I might decide that notrump is okay.
The above paragraph is referring to hands where the responder knows there is no game. For example, you have:
Partner opened a minor, you bid your major and partner rebid 1NT. There are only 2 possible contracts (1NT or 2). You either pass or correct to 2 (100% signoff).
What if you have more strength? For example:
Now, when partner rebids 1NT, you have many possible contracts in mind (partscore or game in hearts, partscore or game in notrump). This is where "new minor forcing" (a popular convention) can be used.
Use/learn this convention only if you and your partner are prepared to study and memorize. Note that this convention should only be used if the responder has a decent (at least game interest) hand. With less than game interest, he either passes the notrump rebid or corrects to his major.
What if responder has 6 or more of his major? That suit is always rebiddable and there is really no need for new-minor forcing (responder knows there are at least 8 trumps). Also note that new minor forcing is used only when the rebid is in notrump.