Opposite a 1 opening, bid a 4+ card major before raising. But with a minimum (5-6 HCPs) and 5-card or longer support for the minor, some will just raise. In particular, the raise to 3D playing Inverted Minors has a maximum preemptive effect.
Minimum support for raising Spades or Hearts is 3, for Diamonds it's 4, and for Clubs you would prefer to have at least 5.
As always, unusual distribution overrides simple point-count, so with 6-9 HCPs, but a long side suit in addition to support for partner, the hand will play much stronger than the HCPs indicate and should be bid accordingly (i.e.: bid the other suit first).
With 10+ points, you can bid other suits before raising partner's major.
Partner opens 1:
Partner opens 1:
Note that after 1-P, entry #240000 will raise Clubs if the hand has fewer than 5 Diamonds, so when you get down to entry #410000, bidding a 4-card suit up-the-line, you don't have to worry about having 4 Diamonds and 5 Clubs in a weak hand.
Your partner may have to open 1 with as few as 3 cards in the suit (in SAYC). If you are weaker than is normally required to bid and you have few Clubs yourself, partner may get passed out and have to play in a 3-2 or worse fit.
If you have a very weak hand and poor support for Clubs (<3 cards), you might bid a new suit with as few as 3 or 4 HCPs. Opener should consider this possibility when bidding again after responder has made a 1-over-1 bid.
If opener's LHO overcalls 1, responder no longer has to "save" opener from a bad fit, of course, and should pass with no fit and a subminimal hand.
In SAYC, bidding goes "up the line". This means that if you have a 4-card major and a 4-card Diamond suit, you bid the Diamonds first. The idea is that if opener has a 4-card major, he can bid it on his next turn and you still find the fit -- a fine idea if opponents don't interfere, which they often do.
In Standard Bridge Bidding For The 21st Century, Max Hardy says that the number one priority is to find a major fit. However, that's all we have to go by from Max because neither this book nor the Advanced version of the book even mention 1-over-1 bidding.
However, some people believe in the idea of finding a major fit so much that they even ignore a 5-card Diamond suit to bid a 4-card major.
In BidBase, the default is to use SAYC, so we do, although we suspect that the majority of experts bypass 4 Diamonds to bid a 4-card major.
Bobby Wolfe said in his newspaper column that with a minimum and five Diamonds and 4 Spades, he would bid the Spades first (responding to 1 because he might not get another chance to bid. However, the logic of up-the-line is that if you bid 1, partner should bid a 4-card major if he has one because you haven't denied one yet.
Even if your major is Hearts and opener bids 1, you don't have to worry about showing your four Hearts because partner has already denied 4 of them by bypassing them to bid Spadeds. You can just bid 1NT over his 1.
When you have 6-7 HCPs and 4-3-3-3 distribution, should you bid the major or 1NT?
Well, if the main priority is to find a major fit, then the 4-card major should be bid rather than 1N. However, few (if any) experts agree with this, and they bid 1N.
However, logic says that bidding 1N will be best only if opener is also balanced and/or doesn't have 4 of your major, and it also wrong-sides the contract.
BidBase will respond with 1 and let opener bid 1N if he is balanced.