There are two sets of mortage interest rates prevailing across the country.

One of them gets most of the publicity on TV, radio and in the press: the average rate on new conventional home loan commitments by federally chartered thrift institutions nationwide. That rate has come down by a percentage point since the late fall, but it's still stuck above the 17 percent mark.

Every month the Federal Home Loan Bank Board announces that figure, and every month would-be home buyers pull in their horns and decide again to stay right where they are. The Bank Board announcement doesn't mention that the percentage of the home sale market actually covered by its monthly survey of mortgages has shrunk dramatically since 1978. That's because S&Ls simply aren't originating many new 30-year loans at 17 and 18 percent and thus account for a shrinking share of the market.

The second prevailing mortgage rate gets little or no formal attention by the federal government, no monthly press releases from Washington. It has no popular name (although the "home buyer rate index" would be a good one).

No government agency or trade association attempts to gauge samples of this rate to compute a nationwide average. But anyone in touch with home sales in major metropolitan markets can tell you that the true mortgage rate being paid by buyers this month is at least four points below the Bank Board's average.

This home buyer rate is the average interest rate actually contracted for, but not documented, in resale and new house and condominium purchases in all 50 states month by month: the subsidized rate loans provided by home builders eager to unload their bulging inventories; the cut-rate loans made between individual sellers and buyers of resale homes and condos; the "resale-refinance" plans offered quietly by S&Ls and banks in numerous cities to get existing low-rate mortgages off their books; and the various other "creative" and not-so-creative finance plans that are behind the bulk of residential real estate transactions in most markets today.

The home-buyer rate index has been trending downward for a couple of months. Not only have rates in the capital markets declined -- allowing sellers to pass on lower costs of money to their purchasers -- but the choice of rate-cutting plans available to buyers has expanded.

Take, for instance, the "buy-down" market. A buy-down involves a subsidized rate for a home purchaser who can't afford (or simply refuses to pay) sky-high conventional mortgage rates.

The consumer's rate is reduced to an agreed-upon level -- say 11 or 12 percent -- by the seller or a third party. The third party could be the purchaser's employer in the case of relocation, or a relative who is willing to help the buyer financially.

The seller or third party deposits a sum of cash with a lender with the instruction that a piece of it be used each month to cut the payments due from the purchaser.

The difference between a $70,000 mortgage at 17 percent and the same mortgage at 12 percent is about $280 a month in principal and interest. The seller or third party's cash covers that difference (minus an adjustment for interest earned on the cash deposits). The monthly subsidy commonly goes on for two to three years, or for as long as 10 years. The precise terms are up to the parties involved.

With greater numbers of builders and individual sellers turning to the buy-down approach in the past six months, the depth of the subsidies open to sharp-eyed purchasers have increased significantly. In other words, the "home buyer rate index" has dropped.

Rather than shallow buy-downs of one or two points, which were common a year ago, buy-downs often go 4 to 6 today. (The so-called "zero percent" mortgage is an extreme variant of the technique -- a total buy-down of the rate. But homes sold with "no interest" carry prices with the subsidy tacked on. That's not necessarily the case with 4- to 5-point buy-downs. Commonly they involve properties where the seller simply needs to sell, and is willing to put less in his pocket in order to close the deal.)

Symbolic of the cuts in the real rates home buyers are paying is the expansion announced last week in the Federal National Mortgage Association's nine-month old buy-down finance program.

Fannie Mae, as it's better known, used to limit the subsidized interest rate on "buy-down" mortgages it purchased from local lenders to 12 percent.

Now, however, Fannie Mae will invest in loans carrying deeper subsidies, well below 12 percent. A home seller or builder, for example, could subsidize a buyer's rate down to 9 or 10 percent and still be confident that his local lender could sell the mortgage to Fannie Mae.

The buyers themselves are even allowed to contribute money to the rate subsidy under Fannie Mae's new "deep buy down" approach -- an interesting alternative to the traditional downpayment.

The point to remember about all this, though, is simpler than buy-down program details: The real rate in the real estate market right now isn't necessarily the one you read about in the depressing headlines.

Ask anyone who's bought a house in the past three months. The odds are 5 to 1 the buyer didn't pay 17 or 18 percent.