The opponents may not be Law-abiding

At the club game yesterday, partner passed, RHO opened 1D and I had a decision to make holding:

S

South

♠

Axxx

♥

xx

♦

Jx

♣

AKQ109

What would you do? You should know that we open nearly all 10-point hands, so partner's initial pass means that we are unlikely to have game our way.

2C seems to be a standout. That's the lead I want if lefty declares NT or any other contract.  It also takes away a bunch of bidding space. The auction continued:

W

LHO

N

Pard

E

RHO

S

Me

—

Pass

1♦

2♣

2♥

3♣

3♥

?

Now, what do you do?

This was how I reasoned.  The opponents, by going to the 3-level, have shown 9 trumps. I think that we have eight, maybe 9. Thus, there are a total of 17-18 tricks on this hand.  Hearts are breaking 2-2, opener most likely has short clubs, so 3H is probably making.  4C, on the hand, may be down only 1. If it is down 2, we will be okay as long as we are not doubled. Consequently, I bid 3S. If partner has 4 spades, we could play there. By issuing a game try, I can probably convince the opponents to not double.  Partner corrected me to 4C and that is where I played, (undoubled).

This was the full hand:

.

Board: 16

Vul: None

Dlr: North

N

North

♠

KQ

♥

Jxx

♦

xxxxx

♣

Jxx

.

W

West

♠

xxxx

♥

KQxxx

♦

Qx

♣

xx

Lead: ♥K

E

East

♠

Jxx

♥

Axx

♦

AKxx

♣

xxx

.

S

Me

♠

Axxx

♥

xx

♦

Jx

♣

AKQ109

.

As you can see, 4C is off 1 with two diamonds and two hearts off the top.  -50 was not a good matchpoint score because 3H is also down -- we'd have gotten 3 spades and two clubs.

The opponents were following neither the law of total tricks nor were they counting losers -- East  had gone to the 3-level on a 8-card fit and a flat, 9-loser hand hand.  . The simplistic rule-of-thumb "3-level is for the opponents" would have served me well.