Alpha is the strong hand's Primary Trump Asking Bid in their own main suit. It asks Responder whether or not they have support for that suit (ie: Hxx or xxxx or better) and also asks about Responder's general level of Controls. "Negative" (1-2 step) responses are negative in terms of trump support regardless of Responder level of Controls, and a "positive" (3+ steps) response agrees the suit as trumps.
In practice, you will find that you use Alpha less often than a low-level Beta combined with a subsequent Theta/Iota. Alpha tends to be reserved for distributional 2-suited hands because it is more effective there, and less effective on single-suited hands. You will decide what fits best with your own experience.
When is a bid Alpha?
Alpha is always the first bid in an Asking sequence if it is used at all. If Alpha is not used to initiate an Asking Bid sequence then it cannot be used later on. A bid is Alpha in the following situations:
A jump-shift response by an unpassed hand to an Opening Bid of 1♦ or 2♣. eg: 1♦-2♥ is Alpha in Hearts.
A 3-level jump-shift by an unpassed hand over an Opening of 1NT. eg: 1NT-3♠ is Alpha in Spades.
A simple new-suit rebid by Opener after a 1♣ Opening and a positive response of 1♥ or 1♠. eg: 1♣-1♠-2♣ is Alpha in Clubs.
A rebid of 2NT by Opener after a 1♣ Opening and a positive response of 1NT, 2♣ or 2♦ is Alpha in the relay suit (because an Ask in the relay is a low-level Beta). Thus 1♣-1NT-2NT is Alpha in Clubs, because 1♣-1NT-2♣ is Beta.
(Complex 1♦ only) A 2-level suit response to an Opening Bid of 1♦. eg 1♦-2♣ is Alpha in Clubs.
There are no other situations where a bid can be Alpha
Responses to Alpha
The stepped responses to Alpha are as follows (the yellow line indicating the divide between negative and positive responses):-
: 0-3 Controls and no good support for Opener's suit (less than Qxx or xxxx).
: 4+ Controls and no good support for Opener's suit.
: 0-3 Controls and support (at least Qxx or xxxx).
: 4+ Controls and support.
: 4 Controls and good support (at least Qxxx)
: 5 Controls and good support
: 6 Controls and good support
Repeat in Same Suit
If Responder gives a negative response to Alpha (ie: 1 or 2 steps), then Opener is allowed to make a Repeat Ask in the same suit. This Repeat Ask is a Theta unless Responder has shown a balanced hand in which case it is Iota. Any Theta/Iota response that shows Qx or xxx agrees the suit as trumps.
If Responder makes a negative response to this Theta/Iota Ask (ie: showing less than Qx or xxx), then Opener may make a Repeat Theta/Iota in that suit to insist on that suit as trumps but otherwise Asking Bids generally lapse. For this reason, with a 2-suited or 5440 3-suited hand, it's better for Opener to Ask in one of their other suits before making a Repeat Alpha (Theta) in their original suit. A negative response to a Repeat Alpha (Iota) in a different suit doesn't prevent a subsequent Repeat Alpha (Theta) in Asker's original suit, but a negative response to a Repeat Alpha (Theta) in the same suit means that Iota in a 2nd suit is then no longer available.
In these situations (ie: a negative response to Alpha) it is often better to hand over the Captaincy to Responder, space permitting, and allow them to bid their hand naturally (ie: to show a second suit, additional length in their first suit, or partial support for Opener's Alpha suit). Moreover, with a decent tolerance or support for responder's suit it is sometimes better to agree it via Gamma rather than using Alpha, and enquire about the Alpha suit afterwards (perhaps with a view to jump-shifting into that suit later on).
Repeat in Different Suit
If Responder gives a negative response to Alpha (ie: 1 or 2 steps), then Opener is also allowed to Ask in a different suit. This Ask is always Iota, whether or not Responder has shown a balanced hand, unless it's Responder's suit, in which case the Ask is Gamma.
This second suit should ideally be at least of 5-card length because the normal rules for Iota apply (ie: a 3-step response showing Qxx or KQ agree the suit as trumps). With a second suit of only 4-card length, it is often better for Opener to hand over the Captaincy, but that will not always be possible or practical (eg: we may be past the level of 2NT already and Opener may have worries about one of the unbid suits). If Opener does hand over the Captaincy and a trump suit is explicitly agreed by natural means, then Epsilons again become available if Responder's level of Controls are or become known (High-level Beta Asks are always available, space permitting).
If the Repeat Alpha (Iota) in a different suit gets a negative response, then the Asker can make a repeat Iota in that suit or even revert to a Repeat Alpha (Theta) in the original Alpha Suit.
After a Positive Response
If Responder makes a Positive response to Alpha, then a "Repeat Ask" is available to check on trump solidity. This Asking Bid is called Sigma, however, and is not always a bid in the agreed trump suit.
Over a 3 or 4-step Response to Alpha: Now an immediate rebid in the agreed trump suit below game level is Sigma.
eg: 1♣ - 1♥ - 1♠(α) - 2♥ - ???. Now 2♠ is Sigma, and 2NT is Relay Beta.
Over a 5-step or greater response to Alpha: Now a Relay in the next suit up is Sigma. Relay Beta not required because a 5+-step Response to Alpha always shows a specific number of Controls.
eg: 1♣ - 1♥ - 1♠(α) - 2♠ - ???. Now 2NT is Sigma in Spades.
For full details on Sigma, please see the Sigma page.
As can be seen from the above, a 3-step response or better agrees Opener's suit as trumps and any subsequent bids in side-suits (unless a jump-shift, which is "to play" or invitational) is either Beta or Epsilon.
The response to an Alpha Ask always affects the range for any subsequent Beta in the sense that the "normal" Beta scale is never used if Alpha has been used. Thus if Responder shows 0-3 Controls in response to Alpha then the weak Beta scale is used, and if 4+ controls are shown to Alpha then the strong Beta scale is used. (If Alpha gets a 5-step or better response, there is no need for a later Beta because Responder's exact number of controls is known).
OCP System Change Log
Sigma after 2NT Alpha or after Interference (16-Mar-2017)
Clarification of when a bid is Sigma: The rationale behind whether Sigma is a Relay or a repeat of the trump suit is based solely on the cheapness of a repeat of the trump suit. After a 3 or 4-step response to a "normal" Alpha (made by bidding the trump suit itself), a repeat of the trump suit is cheap, but over a 5-step or greater response to Alpha, that is not necessarily the case. The fundamental rule for Sigma is that if a repeat of the trump suit is within 2 bids (ie: it's the relay or the next bid above that), then a repeat of the trump suit is Sigma. If, however, it is not within 2 bids of the Alpha response (or any interference over the Alpha response), then the cheapest possible action by Asker is Sigma (include a Pass over interference).
Thus is in the sequence 1♣-1NT-2NT(Alpha in ♣)-3♠-???, now 4♣ is within 2 bids of the Alpha response, so that is Sigma and, in this instance 4♦ would be Relay Beta (and 3NT Natural). Similarly in a sequence such as 1♣-(X)-1NT-2♠(Alpha)-3♥-(4♣)-???, now, because of the interference, a repeat in Spades is not within 2 bids, so now Pass would become Sigma (cheapest action), and Double would be Relay Beta.
Beta after a Repeat Sigma at Game Level (28-Nov-2016)
If Asker uses Sigma after a 3-step positive response to Alpha, and then makes a Repeat Sigma Ask, this will often be at game level rather than below, which means the bid is ambiguous (ie: Teller may treat it as a sign-off if poor for their bidding so far). If Teller responds to the ambiguous Repeat Sigma, they are suggesting they are good for their bidding to date, and therefore we assume they have 2-3 controls rather than 0 or 1. If Relay Beta follows the Repeat Sigma Response, therefore, we retain the Normal Beta scale rather than switching to the Weak Beta Scale (as would be normal given the initial Alpha response). So
2♥-3♣ =Alpha / Support with 0-3 Controls
3♥-4♦ =Sigma / Hxxx
4♥- 4NT =Repeat Sigma / Jack of Hearts held
5♣-5♥ = Relay Beta using Normal Scale / 3 Controls
If the original Alpha response was 4 steps, Teller will almost always treat the game-level Repeat Sigma as such, and any subsequent Beta will continue to use the Strong Scale, as normal.
Sigma was created in August 2007 to cater for enquiries about trump solidity after a positive response to Alpha. Up till that point, Asker had had no way to check on trump solodity after a positive response. (Paradoxically, they could find out exactly what Teller had after an initial negative to the Alpha Ask.