Unusual 2NT and Michaels cue bids - these are discussed together as become more powerful when used together to indicate two suited hands (5+ cards in two suits).
The unusual 2NT is normally bid over an opponents bid at the one level to indicate the lowest 2 unbid suits. e.g. 1D - 2NT shows Clubs and Hearts.
It can however be used as an opening bid (2NT Minor Suit convention) to indicate both minors. This is particulalry useful if used as an adjunct to weak 2 major openings.
It signifies 2 minors as a weak semi pre-emptive bid. Partner would select the better minor.
Using this 2NT convention precludes the use of normal acol 2NT meaning 20-22 points so is ideal when 2D signifies 20-22 regardless of distribution.
This is a variation on the Stayman 2C/2D opening (NOTE - not response to 1NT) where 2C is a general 21-22 and 2D is 23+.
To simplify the bidding, 2C can be used as the 'normal' 23+ and 2D as the general bid of a strong hand.
2 bids then become :
A Michaels cue bid is usually a bid at the 2 level in the same suit as opponents 1 level bid (e.g. 1H - 2H).
If bid over opponents minor, it signifies both majors.
If bid over a major, it signifies the other major and one of the minors.
The response would be to support partner's major if possible - 3 or more cards.
If the cue bid was over a major and the other major cannot be supported, partner can either bid NT requesting the cue bidder to bid the minor, or bid clubs if appropriate, in which cae the cue bidder will bid diamonds if clubs was not the preferred minor.
At this stage the cue bidder's partner can leave the minor or bid the major, whichever is though a better fit.