The tax bill that Congress approved last week is the latest example of what has become a Washington ritual: passing tax breaks on a temporary basis, then extending them temporarily, year after year.

Many of these provisions appear to blink on and off over time, sometimes renewed in the last days of a session, sometimes actually expiring, only to be resuscitated and made retroactive the following year.

And indeed, along with the high-profile credits and rate reductions that got most of the attention, the latest bill includes a couple of dozen items that have been around so long they simply go by the name of "extenders."

These are breaks that range from the research and development credit, which technology companies and manufacturers love, to benefits for oil and gas producers.

But some of them are actually important to average taxpayers. And unlike most of the higher-profile items, which take effect next year, the extenders affect 2004 taxes.

For example, last year and the year before, elementary and secondary school teachers were allowed to take an "above the line" deduction of up to $250 for expenses incurred for the purchase of classroom materials (excluding physical education, which might include -- gasp -- sex education).

"Above the line" means you get to subtract it before arriving at your adjusted gross income. In practical terms, that means you can take it and still take the standard deduction, or if you itemize, this break is not subject to various limitations that apply to normal deductions.

This break expired at the end of 2003, but now is extended through next year. So, teachers, you can start keeping your receipts again.

Also revived last week was the D.C. first-time home-buyers credit. This is a credit of up to $5,000 available to low- to middle-income people who buy in the District. The credit phases out beginning at income of $70,000 for a single and $110,000 for a couple, but the definition of first-time is pretty loose. Essentially, if neither you nor your spouse has owned a home in the District for the past year, and the one you're buying is your main home, you're a first-time buyer.

(A credit is a dollar-for-dollar reduction in tax and has the same value for every taxpayer who claims it. A deduction is a reduction in taxable income, meaning that it saves the amount of tax that would have been imposed on the deducted income. The value of a deduction thus varies with the taxpayer's bracket.)

The bill also extends through next year a provision that exempts a number of personal tax credits from the alternative minimum tax. Absent this extension, taxpayers could see such benefits as the dependent-care credit, the D.C. home-buyers credit and the Hope and Lifetime Learning tuition credits "clawed back" by the AMT.

The bill also eliminates for this year and next the scheduled phase-down of credits and deductions for the purchase or electric and clean-fuel vehicles.

The bill includes a provision that will help some taxpayers escape the creeping clutches of the AMT. This isn't part of the "extender" package -- it hasn't been around long enough -- but unless Congress makes a more fundamental fix to the AMT, it seems set to join that annual parade.

The provisions extend an increased AMT exemption amount -- a sort of big standard deduction used in computing the AMT. The exemption amount this year is $58,000 for a couple but is scheduled to fall back to $45,000 in 2005. The amount for a single taxpayer is $40,250, scheduled to fall back to $33,750.

Under last week's bill the higher amounts will remain in effect through 2005.

There is also a provision that isn't exactly a tax break, but could help a lot of low- and moderate-income taxpayers. The bill lays out a simplified definition of "dependent child," a term that until now had five different definitions in the tax law.

The bill allows taxpayers trying to claim the dependency exemption, the child credit, earned income tax credit, the dependent care credit or head of household status to use the old criteria or to use a new standard one.

The new one is based on whom the child lives with, his relationship to the taxpayer and whether the child has attained a certain age. The new standard isn't truly standard, since different tax benefits have different requirements, such as the age at which each is cut off, but this ought to make it a lot easier to figure out if you qualify.

The new definition takes effect next year.

If all this on-and-off stuff seems to make life difficult for anyone who wants to plan, you're right.

But you've got to learn to think like a politician. Having the cuts expire in a year or two makes them seem cheaper than they really are. That's because budget "scorers" have to look at the law as it is, and the law as it is shows the breaks ending. Thus, 10-year cost estimates for the breaks seem modest, even though everyone knows they are going to be renewed and the real cost will be much higher.

But that isn't the only reason Congress loves this mechanism. Not only does it keep lobbyists and campaign contributors interested, it provides a "gotta-pass" bill each year onto which other, more controversial things can be attached, perhaps pulling them through to passage when they wouldn't have a prayer on their own.

The Internal Revenue Service said last week it is doubling, to $5,000, the amount of business expense sole proprietorships may report and still use Schedule C-EZ when filing a Form 1040 tax return. The change will mean that approximately 500,000 more small businesses -- a 15 percent increase -- will be able to file the simpler schedule, the agency said.

The median after-tax income of a couple with two children was $57,330 last year, up $1,233, or 2.2 percent, from the year before, according to Congress's Joint Economic Committee. Median means the midpoint of a group, with half above and half below.