We open at the 1 level either with a (semi-)natural minor or a 5 card major, and the opponents cheekily make a takeout double. Almost all calls by us, including XX, are transfers.

Transfer to the 1 Level

This basically shows 4+ cards in the target suit, and should be treated effectively the same as a 1 level bid of that suit, including support doubles short Advancer have the temerity to do something. The main difference is that if Advancer Passes and Opener accepts the transfer, that shows exactly 3 card support without extras; and now further bidding is natural (no NMF). Without exactly 3 card support; or with some kind of extra strength or shape; Opener should make the normal rebid over 1 of the target suit.

Example: playing 2/1, with ♠AQ72 ♥KQ3 ♦4 ♣AJT98, after 1♣-(X)-1♥ALERT-(P), Opener would bid 3♠ to show a strong raise.

Transfer to 1NT

Typically this implies a 7-10 HCP balanced hand, although Responder may bid on with extra values. We prefer Opener to declare NT here because that puts the opponents' strong hand on lead, and hides our strong hand.

Advanced Option

After transfering to 1NT, Responder can bid 2♣ALERT to show both minors. 44 over a 1♦ opener, probably more like 55 after 1M.

Transfer to 2 level

This typically implies a negative-free-bid sort of hand; a less than invitational but still constructive hand. If Opener's suit was a minor this will usually be a 5 cards, but if Opener's suit was a major we might reasonably expect 6 cards (although 1♠-(X)-2♦ALERT transfer to hearts is more likely to be only 5).

Over Opener's Major, a transfer, accept, and rebid of 2M implies a normal raise to 2M (example: 1♠-(X)-2♣ALERT-2♦-2♠) except with 3 or 4 bad trumps and indicating lead direction in the target suit.

And, of course, if Responder has a particularly good but offensively oriented hand, they can bid on over the transfer. A new suit would be game-forcing and an old-suit or 2NT invitational.

Michaels

Over Opener's minor suit opener and a takeout double, 2mALERT shows 5-5 in the majors.

Over 2 of Opener's Suit

Bids beyond 2 of Opener's Suit mean what they used to mean: 2NT is a limit raise or better. Otherwise they are weak jumps, unless we are in a fit showing jump situation (e.g. P-1♠-(X)-3♣).

Penalty Redoubles

Since XX is now a transfer, the hand which wanted to make a penalty redouble must Pass (or else make a positive going bid which gives up on a penalty). This is not a problem, because Advancer is pretty much forced to bid something at this point. In fact, this puts more pressure on Advancer because he no longer has the option to Pass to send the message of "no particular suit preference" to Overcaller. So after Responder's Pass, if Responder now comes in with a Double that is for penalty.

Since the penalty nuance of Pass is unexpected for standard bidders, it should be alerted.