Interference

If a 1M opening or a forcing 1NT bid is doubled, the DORA rule applies.
If a forcing 1NT bid is overcalled, the I-DO system applies up to 2 of the opening major.
If a 1M opening is overcalled (below 2 of the opening major),

• bidding 2M shows 6-9 points with trump support;
• cue-bidding opponent's suit shows 10-12 points with trump support;
• double (and then rebidding the major in the next round) shows 13+ points with trump support;
• bidding 3M or 4M shows a weak hand with good trump support;

A summary of all the adjustments (for overcalls, interference and passed hands) is given below:

Scenario	Example	What is 1NT?	What is 2M?	What is 3M?	What is a cue bid?
Original system	1S > (P)	Forcing 1NT (6+) [Next bid 2M = 10-12]	6-9, M support	Weak	----
Responder has passed	P > (P) > 1S > (P)	Natural (6-9)^	6-9, M support	Invitational	----
The 1M bid is an overcall	(1D) > 1S > (P)	Forcing 1NT (8+) [Next bid 2M = 13+]	8-9, M support	Weak	10-12, M support
The 1M bid has been overcalled	1H > (1S) 1S > (1NT)	Forcing 1NT (6+)* [Next bid 2M = 13+]	6-9, M support	Weak	10-12, M support
Responder has passed and the1M bid is an overcall	P > (1D) > 1S > (P)	Natural (8-9)^, stopper in opp.'s suit	8-9, M support	Weak	10-11, M support
Responder has passed and the1M bid has been overcalled	P > (P) > 1H > (1S)	Natural (6-9)^, stopper in opp.'s suit	6-9, M support	Weak	10-11, M support
The 1M bid is an overcall and has been overcalled	(1D) > 1H > (1S)	Forcing 1NT (8+)* [Next bid 2M = 13+]	8-9, M support	Weak	10-12, M support
Responder has passed and the1M bid is an overcall and has been overcalled	P > (1D) > 1H > (1S)	Natural (8-9)^, stopper in opp.'s suits	8-9, M support	Weak	10-11, M support

Two easy rules will help memorise the whole table:

• 1NT is natural if responder had passed before, and is forcing 1NT otherwise.
• Passed hand with no interference is the only case in which a jump to 3M is invitational, because in this case responder cannot use cue bid to show 10-11 points.

Notes:

• A (double) by opponent is not regarded as an overcall (because 'cue bid' is not available).
• If opponent overcalls (1NT), game is generally unlikely. In this case bid 2M for normal support (6+ points) and for 3M weak (preemptive) support; 2NT is a cue-bid (10-12 points). In the rare case of a strong hand (12+ points), a direct jump to 4M can be considered.
• In the * cases, if opponent overcalls at 2-level, then in case of 13+ with M support, responder shall double and then bid the major again.
• In the ^ cases, bid 2NT to show 10-11 (balanced without M support).
• In case of interference by Michael's cue bid (e.g. 1S > (2S)), a double is taken to be the same as 2M while bidding the other major (i.e. opponent's major, 3H in the above example) is taken to be the same as a cue bid.