1NT 'Multi' Overcall

♣  ♥  ♠  ♦

In this text the term 'ANCHOR' suit refers to the lowest unbid suit. E.g Over 1♦ or 1♣ precision the anchor suit is taken as Clubs.

The term '2nd suit' refers to the next highest unbid suit above the anchor suit. Over a precision 1♦ suits are 1♣ anchor and 1♥ is taken as 2nd suit.

The term 'portion' means at least a 3+ card holding in the 2nd suit

Our 1NT! 'multi' overcall is strictly alertable

Using a multi 1NT o'call eliminates using 1NT naturally to express signnificant values  as well as controls  and showing a  somewhat balanced hand.

Our multi  promises a minimum of 9+ HCP. The lowest unbid suit is termed our 'Anchor' suit usually 4 cards. This may  be 3 cards if the rebid, 2D, shows a game going hand. Also if the rebid shows a pre-empt OR a 5-5 the anchor suit is of unknown length.
a) It can be usedonly in 2nd position, that is as a direct overcall of either opponents opening bid of 1 in a suit OR in protective position. [If a minor suit is alerted this has no effect on our 1NT overcall - treating it as a natural  opening but  a Precision 1D is taken as short and Clubs is our ANCHOR suit.]
[We prefer the use of TRAP to defend strong artificial and strong 1♣ openings 16+HCP,  where the 1NT overcall shows either ♦+♠ or ♣+♥ 4-4 or better , and there is another defence for a precision or other 1D !  12-15 HCP opening  or an alerted 1C! 12-15  opening besides a 1NT multi o'call - see this]
 
b) It is artificial & alertable. It can be weak or strong (15+ HCP), single suited OR 2-suited in the lowest 2 unbid suits OR even 3-suited OR some strong hands - of these options the most frequent is the 2-suited usually 5-5 in the lowest 2 suits. If partner holds the ANCHOR suit (the lowest unbid minor) it will offer some comfort!
Nevertheless Partner is required to respond with 2♣ unless the INT overcall has itself been either overcalled or doubled (see below)

c) Its exact nature is only known when the 1NT overcaller rebids.

d) One suit, known as the 'ANCHOR' suit is always known - this is the lowest unbid minor which must be 4+ cards in length. E.g 1♦(opp)-1NT...  will contain 4+ Clubs and if the opponent had opened 1♣ then at least 4+ Diamonds will be held. This is restriction is only for the UK. I see no reason for this rule. We respect in the UK but not elsewhere. Nevertheless when we hold an ACOL style 2♣ opening the restriction of holding 4 Clubs is relaxed.
e) The 1NT overcall is to be considered initially as a pre-emptive bid to block opponents from making a simple 1-level response - this disruption of their bidding has often proved valuable. Subsequently when the nature of the 1NT overcall is exposed partner can make a further decision to possibly compete effectively with the opposition. This may well be the ACOL objection to this o'call as it seriously disrupts ACOL sequences. I say ACOL needs to adapt or die.
f) If opponents minor suit opening is alerted as possibly 0-2 cards then Holding a good 5+ card suit in that minor: we may should make a direct overcall in their suit ONLY when holding opening values and a single-suited hand and the opponents inform that their alerted minor can be 0-2 cards. This approach also has a useful semi-pre-emptive effect on the opponents similar to a 1NT overcall. Partner must realise the significance of the alert and not take the bid as a cue-bid for the Majors.
This 2♣ or 1N overcall are more effective than overcalling in a Major which mostly tends to help opponents in their bidding!

Responder's other options with intervention

Responder must bid 2♣ if his RHO passes. This is just a relay to allow partner to describe his hand.
However if your RHO passes or bids Double and  you possess a 6+ card Major - you may show it with a transfer breakout bid:s: 2♦ for hearts and 2♥ for Spades,
If your RHO doubles partners 1NT o'call you are provided with 2 further options: A 2♣ relay shows 4+ Clubs & ** Redouble shows 10+ HCP
if your RHO intervenes with a suit o'call your usual bid will be PASS. The only exceptions allowed are:

The types of hand declared with a multi 1N O'call and their rebids are as follows:

1. A weak 2-suited hand in the lowest 2 unbid suits at least 5-5 and a 6-7 LTC strict count: 1any(opp)-1N(multi)-2♣(auto response)-3♣ shows 5-5 weak either: ♣♦ or ♣♥ or ♦♥.depending on opps opening bid!
2. A strong 2-suited hand in the lowest 2 unbid suits at least 5-5 and a 4-5 LTC  count: 1any(opp)-1N(multi)-2♣(auto response)-2♥ shows 5-5 strong either: ♣♦ or ♣♥ or ♦♥.depending on opps opening bid! Note 2N is a forcingcontinuation! asking distribution. See 3Club   for sequences following 5-5 exposure. In this particular case we are bidding at a level lower!
3. 3-suited hand - excludes opener's suit 16+ HCP & distribution are: 4441 or 4450.
1any(opp)-1N(multi)-2♣(auto response)-2N shows 16+HCP and the 3 unbid suits. Partner decides the suit to play in.
4. A beta 2♣ opening 23+ Balanced or <=3 1/2.
1any(opp)-1N(multi)-2♣(auto response)-2♦.  Partner now shows cntrls in 3 usual steps: 2♥: 0 control; 2♠: 1|2 controls; 2N: 3|4 controls; 3any: Breakout; 3opps suit(5+ cntrls).
5.  To show a 7+ card pre-empt at the 3-level or longer suit at a higher level.
1any(opp)-1N(multi)-2♣(auto response)-2♠-2N(auto response)-3any' or higher.
Note: in BETA this is the only way to show a long suit (e.g 1S-3D shows a strong 2-suiter D|H !
6. To show a 6+|4 strong hand 16+ HCP andd quality suit:
1any(opp)-1N(multi)-2♣(auto response)-3DHS
 a transfer to 2N then bid your long suit at the 3 or higher level. Our Direct overcall of 4Ma is a not a pre-empt but a 'possible'  distributional  game attempt.
Strong 6+ major & 4 minor (other minor or clubs opps shown with the other Major) .......................3♠|♥|♦ - partner selects denomination Shows 5-5 or better in the suit bid and the anchor suit and a LTC of 4|5 LTC.

Opponents open 1 minor alerted

This alert is simply ignored - initially at least treated as genuine - therefore the other minor is still the Anchor suit.

The following is an alternative approach - (aspects not in use without prior agreement).

A Modified Roman style defence alternative to alerted minor suit openings

Over 1♣! Strong e.g. Precision, then TRAP is employed as standard in Beta.
Over alerted openings of 1mi  e.g  1♦! precision or any 1mi! showing 2+ cards  beta allows a direct cue-bid as a natural 5+ card suit. This is also standard in Beta.

Other responses to artificial alerted minor openings are treated by the concept  of treating an 'unusual' bid with an 'unusual' response - a shortage! Put another way: 'not guiding the opponents' which natural bids usually do. and nevertheless showing a willingness to compete. Reminder: This is played only with agreement.
See Mathematical concept of the 'unusual'  for details - where Hearts become the required Anchor suit!

When opponents intervene before your rebid following a 1NT o'call

All rebids that remain available are still to be used. They always retain their conventional meanings.
If by chance opponents intervention matches your intended conventional reply a Double signifies this.
If you are robbed of your rebid you MUST PASS. However partner may well be able to deduce your probable intention and may bid appropriately!
Essentially nothing changes bidding follows its conventional bids. Even if your intended reply is exceeded by the opponents Most opponent  interventions will be at the 2-level and note well that all your rebids of 2♠ and above are stronger bids and opponents will interfere with great risk to themselves.