Quite an interesting holding

Here's one from this weekend. Dummy holds:
♣ QT8x

You hold:
♣ K9x

Because of the auction you know that LHO (= declarer) has 5-6 ♣s. He leads ♣Q from dummy (fixed typo tnx to Binkley). What should you do?

I had a split second to decide what to do without giving away too much information, and played low because it just felt right. Afterwards I thought about the holding, and realized low was indeed the best. There are a few interesting holdings partner can have:
- ♣x or void: it doesn't matter, you won't make a ♣ trick.
- ♣A: if you cover, you give away a trick.
- ♣J: if you cover, you'll make 1 ♣ trick. If you don't cover, you'll make 1 trick as well, unless LHO decides to play the Ace dropping partner's Jack (but why would he play like that?).
So it only matters when partner has the ♣A stiff.

What about when LHO only has 4-5 ♣s? Then partner can be singleton or doubleton.
- ♣xx or ♣x: doesn't matter, you won't make ♣ tricks.
- ♣Jx or ♣J: doesn't matter, you'll make 1 ♣ trick.
- ♣Ax: again it doesn't matter.
- ♣A: if you cover you give away a trick.
So again, the only situation it matters is when partner has the singleton Ace, and you still shouldn't cover... Nothing changes!

There are even extra (small) advantages of not covering: in some rare occasions, you'll be able promote your ♣9.