2D

Opener has 17-24HCP, with any 4-4-4-1 shape.

Responses

Responder can bid

2H
A relay, with a wide range of hands that need clarification; see below for continuations.
2S
0-5HCP, in principle showing a 4 card suit but may be 3 in a 4-3-3-3 or with a longer minor.

Opener will pass, or bid 3NT for correction with a singleton spade.

With a strong hand and a fit, opener invites game by bidding the suit below his singleton (3S with a singleton Club).

2NT
5-6HCP, good 6-card suit (KQxxxx or KJTxxx).

Opener bids the suit below his singleton, and responder bids his suit at minimum level (but 3NT with Clubs).

With a misfit, opener uses his judgement to choose the final contract.

With a fit, opener can close with a game bid if slam appears unlikely, or cue bid, asking responder to name any singleton.

3 of a Suit
5-6HCP, fair 6-card suit.

See below for continuations.

Continuations after 2D-2H

Opener rebids as follows:

2S
17-20HCP, major suit singleton.

Responder can now relay again with 2NT; opener will rebid

3C
Singleton Heart
3D
Singleton Spade, 17-18HCP
3H
Singleton Spade, 19-20HCP

2NT
17-20HCP, Club singleton.
3C
17-20HCP, Diamond singleton.
3D
21-24HCP, Heart singleton.
3H
21-24HCP, Spade singleton.
3S
21-24HCP, Club singleton.
3NT
21-24HCP, Diamond singleton.

Where opener has shown a 17-20HCP hand and identified their singleton using the responses above (except in the sequence 2D-2H-2S-3D and 2D-2H-2S-3H, dealt with separately above), responder can clarify opener's strength with another relay by bidding the singleton suit; opener will clarify his range by bidding 1 step with 17-18HCP, or 2 steps with 19-20HCP.

A further relay in the singleton suit asks opener to bid controls by steps; 17-20HCP hands bid in steps showing from 4 to 8 controls, 21-24HCP hands show from 6 to 10 controls by steps. After that responder may, if desired, relay again for Queens: opener shows 0-4 queens by step responses.

After the 2D responder is in charge of the auction; any rebid by responder other than 4 of a minor or the relays set out above is to play. 4 of a minor invites opener to continbue to game only if he has two useful honours in the trump suit.

Continuations after 2D-3 of a Suit

Opener rebids as follows:

Pass
Minimum and a misfit.
Game
To play, may be a minimum and a fit, or a maximum and a misfit.
New Suit
Opener has bid his singleton, asking responder to clarify his holding by steps:
1 step
Trumps no better than QJxxxx;
2 steps
Trumps headed by A or K, but no singleton or void;
3 steps
Trumps headed by A or K, with a singleton or void.

In this case opener can cue bid, asking responder to cue bid his singleton.