Basic bridge bidding is based on a practice called "up-the-line", which means that after you open, partner's one-level responses and your rebids are always made in the cheapest of your 4-card suits. For example, if you open 1C and partner has two 4-card majors, he'll always bid 1H to keep the bidding low and give you room to show a possible 4-card spade suit. This assures that you'll find any major-suit fit, and that you'll keep the bidding low until you've exchanged complete information about your suit lengths. (See the lesson on "Which Suit to Respond?" .)
If partner bypasses a suit he could have shown at the one-level, you
should always assume (at least temporarily) that he does not
have 4-card length in that suit. For example:
You open 1D with 
94 
KQ103 
AJ764 
K10
and partner responds 1S. With your minimum opening, it's pointless
to bid 2H (a suit partner has denied), which could force the bidding to
the 3-level. You should rebid 1NT with this hand to show your minimum point-count
and keep the auction low. It's possible that partner has 5 spades and 4
hearts, but in that case, he'll bid 2H over your 1NT rebid.
When you have a stronger hand, though -- such as 4 KQ103 AKJ76 A102
-- you can afford to take the bidding higher. To show extra strength,
you can make a reverse, which is a bid of a
suit partner has bypassed (in the above example, your rebid of 2H would
be a reverse). Put another way, it's a bid that will force partner to bid
at the 3-level if he prefers your first suit. Some typical reverse
auctions are:
Opener Responder
Opener Responder
Opener Responder
1C
1H
1H
1NT
1S
2D
2D
2S
3C
A two-level reverse (where you must go to the 2-level to show your second suit) shows a distributional hand with extra strength, and it forces partner to bid again. Specifically, it promises:
A three-level reverse is made
when partner's response forces you to the 3-level to show your second suit.
Like a two-level reverse, this is forcing and shows extra values. The only
differences are that your second suit will usually be
lower in rank than your first suit, and you may have equal length
in your two suits.
Here are some typical hands:
J107 3 AQJ7 AKQ102
-- You open 1C and partner responds 1S. You can now reverse with a
rebid of 2D. This forces partner to bid again and give you more information.
If he rebids a long spade suit, you'll raise to 4S. If he bids anything
else, you can further describe your hand by bidding spades next. Note that
since you didn't raise spades right away, you denied holding 4-card support.
Your belated raise shows 3-card spade support and , by inference,
tells partner you have a singleton heart (since you've shown 9 cards in
clubs and diamonds and 3 cards in spades).
AQJ106 J KQ1076 A5
-- You open 1S and partner bids 2H, showing 10+ pts. and at least 5
hearts. Since you must go to the next highest level to show your diamond
suit, a 3D bid here is a 3-level reverse (often called a high-level
reverse). With a weaker hand (12-14 pts.), you would rebid 2S to
show a minimum and keep the bidding low.
AK1054 KQ9862 A4 Void
-- You can also use a reverse bid when your suits are longer
than 5-4. Here, you open 1H, partner responds 1NT, and you make a
reverse bid of 2S to force. Over whatever partner bids, you'll bid your
spades again to show extra length. Partner will know that you must hold
5 spades (because you bid them twice) and 6 hearts (because your hearts
must be longer than your spades). Partner can now decide what the trump
suit should be.
K9 AQ72 AQ KJ754
-- You open 1C and partner bids 1S. This hand qualifies as a
reverse to 2H, but a 2NT rebid is a better description of your strength
and distribution. Your hand is fairly balanced, your suits are not robust,
and you have almost half your points in your doubletons. If you instead
bid 2H here, you promise only about 17+ pts., and you tend to show a hand
that's more suitable for a trump contract than for 3NT.