Selling one house depends on the buyer’s selling another house, and that deal in turn depends on yet another sale, and so on and so on.

A failure to get a mortgage approved at any stop along the way can halt the sales of an entire series of homes.

The loss of mobility stems from the fact that homeowners with houses worth less than they owe can find themselves unable to move to accept a job in a different city.

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That has been true when home prices have declined in the past. What is new this time, and could bring much greater pain and more selling pressure in many markets, is the existence of “exploding mortgages,” loans with monthly payments that will rise sharply within a year or two.

Homeowners who can refinance such mortgages are doing so now, but others — without enough income to qualify for a new mortgage or with a house not worth what is owed — may be forced to sell if they cannot get concessions from their lenders.

That threatens a downward spiral in prices, most likely in areas where prices rose the most and where unconventional mortgages were most often used. Broadly speaking, that means many areas on the East and West Coasts, plus once-hot areas like Arizona and Nevada.

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Just how bad it will be depends in part on how the banks fare. Those with money to lend will be able to charge more than they formerly did — and thus get better profits.

That gives a leg up to banks that can finance themselves with low-cost deposits. Bank of America, in agreeing to acquire Countrywide Financial, purchased a mortgage distribution network that could no longer be financed as it had been. But the more home prices fall, the greater the risk that banks will have to write off more losses, resulting in steep drops in capital levels and even causing failures of financial institutions — thus reducing the supply of available loans.

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That reduction of supply is particularly threatening to buyers of more expensive homes. Jumbo mortgages, those over $417,000, cannot be sold to Fannie Mae and Freddie Mac, the government-sponsored enterprises that are still buying loans. There is legislation pending to raise that limit in some parts of the country, but it appears that many such loans were so risky that they cannot be refinanced. The only way prices got so high was that people who could not afford to buy those homes were given mortgages they could not hope to repay unless home prices kept rising.

It is possible that we will see a negative spiral, in which lower prices reduce the availability of loans, and thus push prices lower. That, in turn, could leave more homeowners unable to refinance exploding mortgages, producing more forced sales that push prices even lower — and further reduce bank capital.

That would be the reverse of the cycle that prevailed until mid-2007, when easy credit made loans available even to those with dubious credit, driving up prices and, for a time, making it appear that there was little risk in mortgage lending.

At some point, the pressure on the government to step in will be intense. Its efforts so far have been mixed. Tougher lending standards ordered by regulators may only increase defaults, but efforts to persuade banks to renegotiate loans may reduce the dislocation for some homeowners.

But barring a huge government program, the housing decline may be prolonged and bitter for nearly all involved. The worse it gets, the bigger the losses for financial institutions and the less able they may be to make the loans to turn things around.