[mikl_plkcc]

I sat at East: ♠JT842 ♥KT32 ♦ ♣AQ63

We use Multi Landy for 1NT opening defence. Bidding went
1♦ - (1NT) to me, where 1NT was natural.

I didn't know if Multi Landy should be used here. Our agreement was that if WE overcall 1NT, we treat it as the same as 1NT opening (system on), but we hadn't discussed if THEY overcall 1NT yet.

I made a X, and hope that partner would either pass it or bid his second suit, which I would pass. He pulled it to 2♦ with the following hand, which I thought that he would have at least 6 or 7 diamonds, single suited:
♠K763 ♥98 ♦AQJ98 ♣J9

We ended up playing 2♦ with 5-0 trump fit and lost a large amount of IMPs.

How could we improve the bidding?

[smerriman]

"Standard" is that double is penalty, and everything else is natural and weak. You can play something else, like 2♣ being the majors, but can't afford to lose a natural double.

[mycroft]

With one partner I play (and forget sometimes) defence on over 1NT overcalls. But that defence has a penalty double, necessary as smerriman says.

It works; I play it because she insists (and she plays fit jump shifts even out of competition because I insist); I wouldn't suggest it to any other partner. I don't think it's bad, just not better enough to matter.

[apollo1201]

Most common follow ups after they overcall 1NT over a minor are:

X for penalties, 9+ (with a good lead), not too unbalanced

2C (or 2m) for majors, 4-5 at least, and from 3 to 8-ish depending on shape and vulnerability (I am not passing Qxxxc JTxxxx at green, but 44 M on the upper range could be ok).

The rest natural or transfer (to gain a 2nd round of bidding, but the downside is that LHO has 2 opportunities to bid and we offer a cue bid as well)

Avoid X with unbalanced because opps could run the minor partner opened and make 7 or even 6 tricks overall while you can score a good partial or game in a suit.

Here, partner should pass your X, esp with only a 5-cd D, suit but I would not have Xed for the reason I mentionned. I would have bid 2C for majors, partner bids 2S and you can try 3S and partner might bid 4S depending on feeling. It is not lay down for sure but I have seen more desperate contracts.