As a general rule, we shouldn't raise to the two level if we hold less than 9 points. If we can't support partner's suit and hold another suit that we are able to bid at the one level then there isn't usually a problem - we bid it. If we don't hold a suit that can be bid at the 1 level, then we bid 1NT. As a responding bid, 1NT doesn't tell partner we have a balanced hand, it just tells them we have 6+ points and so have to make a bid.
Look at this hand which we hold when replying to partner. Partner has opened with a bid of 1 spade.
Spades: 8 7
Hearts: Q J 8 3 2
Diamonds: Q 7 5 3
Clubs: J 6
Ideally we would like to bid 1 heart, but as partner has already bid spades we can't bid hearts without raising to the two level. We have to bid 1NT.
Are there any circumstances where we can bid at the two level even if we hold fewer than 9 points?
This is where we use the:
Rule of 14.
This rule is used in one circumstance only - when replying to partner's opening bid of 1 of a suit and when holding 6 - 8 points and when our longest suit has a lower rank than the suit bid by partner. Add up the number of points in your hand and then add that to the number of cards in your longest suit.
If the total = 14+ then we can use the Rule of 14 and make our bid at the two level.
In the previous example that would give us a total of 6 (high card points) plus 5 (cards in longest suit) = 11. The hand doesn't satisfy the Rule of 14 and should be bid as 1NT.
Some More Examples
S: J 8 6
H: 10 3
D: K 6 4
C: K J 10 8 2
Partner opened the bidding with 1H. Ideally we would bid 1C, but that is a lower rank bid than partner's opening so we would have to raise to the two level. If we use the Rule of 14 we get a total of 13. We have 8 points in hand and 5 cards in our longest suit. The total is less than 14 so we bid 1NT.
S: 9 4 2
D: J 8 5
C: A Q 9 7 3 2
Again, partner has opened the bidding with 1H. Ideally we would like to make a club bid, but to do so would have to bid at the two level. We apply the Rule of 14. We cold 8 points in hand and we have 6 cards in our longest suit. This gives us a total of 14, so this time we satisfy the Rule of 14 and can bid our clubs at the two level.