When does a banking crisis turn into a real crisis? The textbook answer is when banks start going bust, or people think they are going bust and try to extract their money in a hurry.
Last week I passed a curiously long queue snaking out of the door of a branch of Abbey, the one-time building society now owned by the Spanish Santander.
It was so long that I went in to take a look. Sure enough, there was only one clerk at the cash counter - something that does happen when a bank wants to slow things down.
Still, Abbey is notorious for queues when there is nothing wrong at all, even more so than NatWest. Anyway this one had evaporated a couple of hours later.
We don't have that kind of banking crisis. What we do have are banks fearing that one of the others may suddenly turn out to be up to its eyebrows in dud American mortgages. So they hesitate to lend to each other and charge heftily for the supposed risk when they do.
The Bank of England moved yesterday to free up the overnight money market, in which banks borrow from and lend to from each other to straighten out their books. This was a mechanical tweak of the system. It will not save any bank in real trouble.
The lesser moment when bank troubles turn into real trouble is when nervous bankers stop lending to the rest of us or charge more than we can afford for the money so that we don't borrow it. That was what gave stock markets on both sides of the Pond a fright yesterday - news of American house purchases falling through at the last minute and mortgage lenders backing out of deals they had agreed before. That may be ominous. Yet it would be more so if American mortgage lenders were still pumping out cash to people who can never afford to pay it back.
Real trouble starts when the prudence gets overdone. Grossly over-ambitious private equity artists may weep into their gin because they can no longer borrow dirt cheap billions to bid for mighty multi-nationals. But nobody else will.
Other corporate lending is still going ahead. Carillion tells how it asked for a £500 million facility and banks oversubscribed it and put up £590 million.
That is Irish miles short of the recession-inducing credit crunch that Jon Moulton was talking about up yesterday. Something like that may indeed happen. So far, though, it isn't happening. Until yesterday the stock market was taking lively comfort from the absence of distress throughout the real - non-banking - world.
Yesterday it changed its mind, abruptly, as stock markets do. This is a drama starting in the American housing market. It will be resolved only when we discover whether falling American house prices, lack of reckless American mortgage lending and the wilting borrowing power of American home-buyers, really does stall the world economy.
My hunch is that there is a decent chance that it won't.