Fourteen of you have written to scold me for not being more precise about the scam that turns $1 bills into phonies that look like $20 bills.

"How do they do this?" readers want to know. "It is not like you to be so fuzzy about details," one said.

Permit me to remind you that I write this column on a video display terminal rather than on a typewriter. Some VDTs may be capable of reproducing blueprints for crime, but mine is not.

Readers were given enough information to alert them to the activities of crooks who can change a single into what appears to be a twenty, and were warned that if they accept an altered bill the loss will be their own. That's as far as I intend to go in describing the method of operation. I am not running a school for crime.

Meanwhile, Ben Silver offers an observation that may help you stay out of trouble. Ben has been a cashier for United Drug Service for about four years and says he has "on many occasions" spotted altered $1 bills that were supposed to be mistaken for $20 bills, and has declined to accept them.

Ben agrees that it would be useful if people memorized the pictures that are on Federal Reserve notes of various denominations: Washington on the $1 bill, Jefferson on the 2, Lincoln of the $5, Hamilton on the 10, Jackson on the 20, Grant on the 50 and Franklin on the 100. But there's another checklist.

Every denomination except one has a picture on the reverse side of the note. The exception is the $1 bill -- the one used by the cheaters because it is the cheapest authentic prop they can use in their scam. The back of a $1 bill shows the Great Seal of the United States, obverse and reverse, with the two reproductions separated not by a picture but by the word "ONE" spelled out in large letters. There is no picture on the back of a $1 bill.

What's on the backs of the other bills? The modern $2 bill reproduces the famous painting titled, "Signing of the Declaration of Independence." Lincoln's 5 has the Lincoln Memorial on its back, Hamilton's 10 shows the Treasury, Jackson's 20 the White House, Grant's 50 the United States Capitol, and the back of Ben Franklin's C-note shows Independence Hall.

Denominations of $500 and $1,000 are no longer circulated. When Sam Snead removes a bill of this size from one of the tomato cans buried in his back yard and asks his bank for change, the bank gives it to him -- but the Federal Reserve retires the bill at once. PERSONAL NOTES

Brother Robert Matthews-EL, Lewisburg, Pa.: The Washington Post does not own WTOP. Our staff has no control over what WTOP says or does. I am sorry to hear that you find conditions in Lewisburg deplorable, and I hope remedial action will be taken. I have noted your criticism of my suggestion that we teach people that narcotics are bad for them, and I have a question to ask: Do you think it might help if you devoted your energies to teaching people that Lewisburg is bad for them? Perhaps if you told them how bad things are in Lewisburg they'd have a greater incentive for staying off narcotics and out of jail.

Howard White, Oxford, Ohio: I did not attempt to compile a representative list of the products made from crude oil because there are so many, not the least of them the plastics to which you refer. I just wanted to remind readers that a gallon of crude oil does not translate into a gallon of gasoline.

Bill Sheriff Jr., Silver Spring: I am flabbergasted to learn that many of my readers do not understand quotation marks, or perhaps fail to see them. Quotation marks around a sentence -- or, more to the point, around a few isolated words -- indicate that the words inside the quotation marks are somebody else's, not mine. Yet many readers criticize me for words I quote, which is a mild version of shooting the messenger who has brought unpleasant news. I am particularly dismayed to discover that so many of my colleagues in the writing business don't see or don't understand quotation marks. We may need a new punctuation mark that's a better attention grabber.

James D. Magness, Greenbelt: The need for money at Children's Hospital is year-long. Our fund campaign is not designed to buy Christmas toys but to provide continuing help for poor children who are sick. Send your check to Children's Hospital Fund Raising Office, 111 Michigan Ave. NW, Washington, D.C. 20010. Thank you. OH, THANK GOODNESS!

One of the most startling headlines I've seen in our newspaper recently appeared in Sunday's editions. It said:

"All Utah Condemned to Face Firing Squad."

Fortunately, the story beneath that headline explained that prisoners condemned to death in Utah will no longer be given a choice between hanging and being shot.