Under the new tax law, yesterday's average price of a stock or bond becomes the historical benchmark for calculating the amount of income tax individuals must pay in the future on securities they inherit that were urchased before Dec. 31, 1976.

If a stock averged $50 yesterday and passes into the estate at $70 while the person who inherits it waits to sell until it is worth $90, the seller must pay a capital gains tax of $40, the difference between yesterday's average price and the selling price. Under the old tax law the individual would have calculated capital gains as the difference between the $70 and $90.

For purposes of the tax law, the price of the stock or bond is not the closing price but an average of the day's high and low.

In today's editions, The Washington Post is carrying a complete listing of the day's trading in New York Stock Exchange bonds in addition to the day's trading in NYSE and Amex stocks and options.

On Sunday, as usual, The Post will carry complete lists of stocks traded during the week on the NYSE and the Amex, including the year's highs and lows. On Sunday, The Post will also carry its regular list of mutual fund and bond quotations for the week.

On Sunday, Jan. 9, The Post will publish its annual economic outlook section which will carry a complete rundown on 1976 trading in New York Stock Exchange stocks and bonds, American Stock Exchange stocks, and mutual funds in addition to the regular weekly tables encompassing trades for Jan. 3 throug Jan. 7.

For purposes of the 1976 tax law, if a stock or bond (or other marketable security) did not trade on Dec. 31, its benchmark price should be derived by averaging the price on the nearest trading day before and the nearest trading day after Dec. 31.