Delta is a primary trump asking bid in Opener's trump suit and always promises a very strong hand, ostensibly 24+ points but this can be shaded with a very powerful and distributional 2-suiter with exceptional playing strength.
The criteria for using Delta should not be devalued far, however, because it's use is game-forcing and initiates an Asking Bid sequence even when Responder may have an extremely weak hand with no fit for Opener.
Delta is therefore generally only used when Opener has game values in his own hand alone and is so strong that they can afford the space used by Delta and its responses.
When is a Bid Delta?
Delta is used in only one very specific situation:
A jump-shift immediately over a negative 1♦ response to 1♣ (or, when opps intervene over the 1♣ opening and Responder either passes, doubles or redoubles, an immediate jump-shift over the pass or double).
There are no other situations where a bid can be Delta. Bear in mind, however, that Theta uses responses that are identical to those of Delta, and that Iota is a contracted Theta. Delta is not available over a positive response to 1♣ (in which situation a jump-shift is Zeta, not Delta)
Delta is the only way that Opener can generate an Asking Bid sequence after a Negative Response to 1♣.
Responses to Delta
The responses to Delta are as follows (in all cases, honour cards shown are generic rather than specific):
: - or x
: Q or xx
: Qx or xxx
: KQ or Qxx
: KQx or Qxxx
: xxxxx or xxxxxx
: Qxxxx or Qxxxxx
: KQxxx or KQxxxx
Repeat Delta Asks
(1) In the same suit
Whatever Responder's initial response to Delta was, immediate Repeat Delta Asks in the same suit are available below game level and these ask for clarification of Responder's exact holding. Where Responder has shown two alternatives (ie: not a 5 or 7-step response) then the shorter holding is shown as the 1-step response to the repeat Delta Ask. Opener should not normally need to make a repeat Ask over a 5 or 7-step response to the initial Delta. If they do, however, then this is just asking about possession of the Jack in addition (and the 1-step response denies). A repeat Delta in the same suit always agrees the suit as trumps, whatever length Responder originally showed and whatever their response to the repeat Delta.
(2) In a different suit
In the event of a response to Delta of less than 4-steps (ie: e negative response) immediate repeat Asks in a different suit are Iota and are uncondtionally forcing, even at game level. The responses to Iota are similar to those for Delta except that steps 1 & 2 are combined into a 1-step response of "Less than Qx or xxx". If Responder made a positive response to Delta (ie: 4-step response or better), then the new suit bid is Epsilon, of course, because the Delta suit is now agreed as trumps.
With Delta a 4-step response agrees the suit. Although Delta is normally used only with 5-card or longer suits Opener may sometimes be forced to use it with a 4-card suit (eg: a 24+ point 4441-shape hand) and should bear in mind that Responder will agree the suit with only 2 honours doubleton or with 3-card support with one top honour.
Handing over the Captaincy
If Responder's declared holding in the suit is insufficient for trump purposes, then an immediate bid in No Trumps over the response to Delta hands over the Captaincy and Asking Bids lapse. For practical purposes this will always be at game level or higher (except for the sequence 1♣-1♦-2♥-2♠-2NT) and Opener should be aware that the action of handing over the Captaincy at game-level or above is not forcing in this instance. Similarly, Responder should be aware that Opener has an extremely strong hand and not settle for, for example, 3NT if they have a near maximum negative and there is a slam potentially in the offing elsewhere. Handing over the Captaincy at the level of 2NT (ie: as in the sequence shown above) is forcing.