Used over opponents notrump openng.

Double = Unspecified one-suiter, relays responder to 2C
2C = Clubs and another suit
2D = Diamonds and a major
2H = Hearts and Spades
2NT = Major-minor two-suiter, very strong hand hand (6-5 or better)
(This relays partner to 3C, over which opener identifies
his two suits via a fairly complicated code.)
3NT = Two-suited in the majors, very strong hand

2S = Spade one-suiter, 6-card suit or strong 5-carder
3-bids = Long suit (7+ cards), preemptive

Responder bids 2C to let the doubler show his suit. The doubler can
pass if the suit was clubs; otherwise the doubler bids the suit. If the doubler holds a
strong hand with game interest, he can jump to 3 of the suit. Responder is not forced to take the 2C relay if he has his own suit (a
6-card suit or good 5-carder). After the overcaller shows his suit, 2NT by responder shows a good
game invitation. A raise shows a fit, but not enough for a "serious" invitation.

Responder can pass or ask for overcaller's second suit by bidding the cheapest suit (2D over a 2C overcall, etc.). If responder "skips" the cheapest suit to bid another suit (i.e., 2H over a 2C overcall), responder is showing his/her own 6-card suit or strong 5-carder. 2NT by responder is a strong game invitation and asks opener to bid the other suit at the 3-level. If responder asks for the second suit and then bids 2NT, it's a strong game invitation in the second suit. Raises of minors tend to be preemptive. Raises of majors are mildly invitational, since 2NT is available to show a good invitational hand.


If the opponent bids or doubles directly after the DONT overcall, responder may continue to inquire into the bidder's suit(s)
The general meanings of responder's bids are:
Pass = play in the current contract
Redouble (over a double) = asks overcaller to bid the second suit
Double (of a natural bid) = asks overcaller to bid the second suit
This double shows some high cards and tolerance for all unbid suits.
Double (of an artificial bid) = lead directing
[ some partnerships have the double over the artificial bid mean the same thing as the double over opponents' natural bids]
Any suit bid = responder is showing his/her own long suit