The great movement for tax reform peaked, understandably enough, last April 14 when millions of law-abiding Americans were spotted scurrying around their houses looking for evidence to support their deductions. Most of what they were saying about the IRS was unprintable, but the words "fair" and "unfair" kept coming up, and those words will keep coming up again in the renewed great movement for tax reform.

One person's fairness, however, is another's subsidy, which is an elemental fact that is going to make this whole business very tricky. And nowhere is it going to be trickier than in the matter of what's fair for families.

The so-called traditional family, in which one parent works and the other stays home and cares for the children, accounts for about 10 percent of American households. Nearly 20 percent of households are headed by single parents. As of 1983, more than half of all married couples were two-income households, making the working couple the norm.

All families, even if they are of the same size and income, are not taxed equally.

Under current tax code, for example, single heads of families pay more than married couples. This happens because of a disparity in the zero bracket amount (ZBA) -- the first dollars of income that are shielded from taxation. According to Parents Without Partners, the zero bracket amounts were the same for every family until 1975. Then the amount for married couples began rising while the amount allowed single parents remained the same as the amount for a single person.

The result, according to Parents Without Partners, is that married couples, whether or not they have children, can shield $1,190 more from taxation than single parents in the same income bracket and with the same number of people in the family. Thus, about 8.5 million single parents have to pay more taxes than married couples.

Various tax reform proposals bump that zero bracket amount up in varying degrees. In testimony before the House Ways and Means Committee in June, Rep. Patricia Schroeder (D-Colo.) argued that "the principle of the ZBA is to allow for a minimum amount of tax-free income for basic living expenses. It should be set at an amount that bears some relation to reality. Presently, the ZBA does not even approximate reality for single heads of households. It is set at $2,390, the same as that allowed single persons . . . .

"The single head of household family with one or more dependents has the same household expenses as a same size family headed by a married couple. The single head of household family has only one earner (average income of $11,789), whereas a married couple frequently has two earners (average income of $27,286). The single head of household family should have the same zero bracket amount as a married couple." The Reagan administration's tax plan has called for raising the zero bracket amount to $3,600 for single heads of households, $400 short of the level for married couples.

"I encourage you to close up this gap," said Schroeder. "If your purpose in tax reform is to be fair to families, you must treat similarly situated families the same." A bill introduced by Rep. Barbara Kennelly (D-Conn.), a member of the House Ways and Means Committee, would do that.

The same argument should be made for continuing the relief Congress gave married taxpayers from the so-called "marriage penalty." The relief comes in deducting from taxable income 10 percent of the wages of the spouse with the lower income. The administration wants to scuttle that. Testifying at the same hearing, Rep. Olympia Snowe (R-Maine) said: "Without this deduction, a two-earner couple where each spouse earned $10,000 a year would have paid 15 percent more in federal income taxes in 1984 than two single taxpayers with the same incomes. With the deduction, however, the penalty was reduced to 5 percent.

"This deduction was passed by Congress, not to establish a preference for two-earner couples over one-earner couples in the tax code, but rather, to reflect the changing circumstances of American families' lives." More than half of all families with incomes between $25,000 and $40,000 have two earners, she said.

Two-worker families have all the expenses of maintaining two jobs, just as single-parent households have all the expenses of maintaining a home. Family structures have changed, and someone living in a single-worker household this month may be in a two-worker household the next month, and could be a single parent the next year. Genuine tax reform will recognize the different kinds of families we have now and ensure that each gets a fair shake simultaneously.