1♣ = 2+clubs.
Either any balanced hand (11)12-14 HCP or 18-20- HCP (without 5♦) . ≈ 70%
(with 5♦-4 ♥-2-2 or 5♦-4 ♣-2-2 you may open ♣ or
1NT.
Or 5+♣ unbalanced (except 4-4-4-1♦ ) 10-23 HCP (10 HCP with a solid suit). ≈ 30%
(In the 20-23 HCP range, hand
is two-suiter)
The developments are very similar to those made after an overcall on
1♣
opening.
1♦ = 4-5 hearts (may be very weak). (6 hearts if invitational or with 4 spades).
1♥ = 4-5 spades (may be very weak). (6 spades if invitational or with 4 hearts ).
1♠ = No major. (XnT, 2♣, 2♠, 3x not possible).
If unbalanced, promises 4+ diamonds and is limited to 11 HCP. It replaces STANDARD bid of 1♦ . It puts a strain on opponent's side : no take out double for major and they have to go to 2 level to bid a major.
1NT = 10 + -11 HCP , interest(s) receiving the lead otherwise 1♠.
Superior to STANDARD. The auction
allows to stop at the 1 level if the opener is limited
to
12 HCP. It used to declare the game with 10 + HCP opposite 14 HCP.
2NT = 12 (13-) HCP , interest(s) in receiving the lead otherwise 1♠.
3NT = 13-15 HCP (13+), interest(s) in receiving the lead otherwise 1♠.