Husbands and wives should sit down with a lawyer and take a look--all over again--at their jointly owned property. Under the terms of the new tax law, you may see some reason to put certain joint property into separate names.

There are three key rules to take into consideration--two of them new with the 1981 tax law and effective in 1982; the third, an older rule with new possibilities.

Rule one: With jointly held property, each of you will be presumed to own one-half. Under the old law, joint property was assumed to belong to the first of you to die, unless the survivor could prove that he or she paid for it.

Rule two: Property given back and forth between husband and wife will no longer be taxed as a gift. So you can now rearrange your property ownership without tripping over the Tax Code.

Rule three: When a person dies, all the capital gains--on his stocks, real estate, business interests and other investments--escape income tax. No income tax will ever be paid on those gains, by the estate or by the person who inherits. This long-time rule was thrown out of the Tax Code in 1976 but restored in 1978. Properly used, it can save a family a lot of money.

Taken together, these rules offer some interesting possibilities for married couples.

Take the typical situation of a jointly owned family home. If the husband dies, the wife inherits it automatically.

The house has probably gone up in value since it was bought. Thanks to rule three, explained above, the husband's death nullified any income taxes owed on his half of that gain in value.

If the wife sells the house immediately, she will owe a capital-gains tax only on the profit arising from her half of the house. The half she inherited from her husband escapes income-tax free.

Now take that same house, but put it solely in the husband's name. If he dies and leaves it to his wife, she will escape income tax on the entire profit built up during his life.

If the husband is older than the wife, or in worse health, the family may save some money by playing the mortality statistics. If he owns everything, and leaves it to her, his death becomes the ultimate tax shelter. It effectively wipes a lifetime of capital gains off the books of the Internal Revenue Service.

In the case of a family home, eliminating joint ownership may not make much difference to how much tax the survivor ultimately pays. If the house is sold, the wife can buy another house and defer the tax. At age 55 or later, she can tax-shelter a profit of up to $125,000.

But other forms of property are another matter. There may be a good deal of money to be saved by putting appreciated stocks, real estate, or shares in a family business into the hands of the person--husband or wife--more likely to die first. (This depends, of course, on there being no more changes in the estate-tax law, warns New York lawyer Edward S. Schlesinger.)

Naturally, where there's a loophole like this, there are abuses. In the past, for example, a son might give his dying father appreciated stock, then get it back in the father's will--with the capital gains liability conveniently erased.

Under the new tax law, this two-way transfer will work only if the "deathbed volunteer" lives to hold the property for at least one year. Also, this free ride is not without limits. If the estate owes taxes, the transferred property will cost more in estate taxes than would have been owed in capital gains taxes.

(The old game is still effective, however, if the appreciated property is returned to someone other than the original owner or his spouse. In the above example, the capital-gains liability would be eliminated if the wife's stock were given back to their child--although estate taxes still have to be considered.)

Community property gets a far better break than joint property. When one owner of community property dies, the capital gains tax is forgiven on the property's entire value--the wife's half as well as the husband's. This may encourage more people to retire to one of the eight community property states (Ariz., Calif., Idaho, La., Nev., N.M., Tex., and Wash.).

Two warnings: By giving up joint ownership, a wife may save herself some taxes if the marriage survives. But she may imperil her financial security if the marriage fails. At divorce, most states now divide marital property between the spouses. But some states still award the property to whomever holds formal title.

There are many other implications to joint or separate property, relating to state property and tax law, estate taxes, trusts and other matters. Don't mess around with property ownership before getting good legal advice.