"I've got a scam to report," said the voice on the phone.

"Scam Reporting Bureau open for business," I said, reaching for a yellow pad.

"I live in Montgomery County, Maryland," said the voice. "I work in the District. I buy a copy of the Post in either of those jurisdictions, it costs me a quarter."

"Costs me a quarter, too," I said, helpfully.

"But I was out at National Airport the other day and I bought a copy at the newsstand and they charged me 26 cents. I figured this had to be some kind of rip-off. So I said, 'What is this? The Post only costs a quarter in Maryland and D.C.' And they gave me some rigamarole about taxes in Arlington County. You better check this out."

Well, I did, Voice, and I discovered a creature called the Retail Sales and Use Act. It covers not only Arlington County, but the whole state of Virginia, and it has been in effect since 1966.

The act provides for a one-cent tax on periodicals that are issued at regular intervals of three months or less and sold at newsstands. If you buy the publication from a dispenser box, or if you subscribe by mail or home delivery, the tax doesn't apply.

By the way, Voice, the guys who run the newsstand at National have heard your suspicious question so many times that they've committed the answer to memory.

As soon as researcher Linda Josephson called to ask about The Mysterious Extra Penny, they referred her without a fumble or a delay to Virginia Code 58-441.1, section 1-73.