I read some years ago that about two-thirds of all U.S. paper currency is in circulation overseas. Don’t know if that’s still true.
How do those pens work, anyway?
It was true in 2003:
Not only is it true as of the last survey, but the most common bill is the $100 bill. If you live in a country where the currency is a wee bit on the rickety side (“Should I buy toilet paper, or just directly use the paper currency?”) find that hoarding much of their cash in dollars is the best investment. This is especially true if your country is the type to suddenly declare large denominations of bills as invalid, or to seize large bank accounts without any explanation.
One of the interesting things is that the Euro is competing with the dollar with these hoarders. The Euro comes in €100, €200, and €500 notes. It is a lot more convenient to store $5000 in ten €500 notes rather than 50 $100 bills.
They use an iodine based ink which reacts to any starch in the paper. On the cloth based paper real bills, they barely make a mark (if any). On wood based paper used in many counterfeit notes, the iodine leaves a dark stain.
These are mainly used to catch people who are using copiers to print out copies of bills and trying to pass them off. They should be able to catch people who are using rice paper which looks and feels like the cloth paper used in real bills.
So, when they say ‘overseas’ do you think they really mean “out of the country”? 'Cause I’m willing to bet that a LOT of US cash is in Canada, Mexico, Central and South America… and that’s not exactly Overseas… I don’t figure if you can drive to it you’re going overseas.
Yes, I should have written “out of the country.” Outside of U.S. territory.
You could take a boat to there.
There are still people in Alabama and Mississippi who will insist they don’t belong to the same country as New York and California.
(And, there are people in New York and California who wish it was true.)
Of course, that’s true from Boston to Miami, too. 
[QUOTE=qazwart]
They use an iodine based ink which reacts to any starch in the paper. On the cloth based paper real bills, they barely make a mark (if any). On wood based paper used in many counterfeit notes, the iodine leaves a dark stain.
[/QUOTE]
Yes, and to show how stupid an anti-counterfeiting technique this is, people have done demonstrations. Spray laundry starch (for ironing clothes) on your real dollars and they will show up as fakes. Coat your newsprint with hair spray, and it will not show starch.
On a side note, I always thought “overseas” was pretty much British usage due to its widespread presence in the local English-language press, which largely follows British usage. (It’s a mix really but mostly British style.) But then I recall there was one thread on the Board asking about the use of “overseas,” and it seems like most posters, even the Brits, said they didn’t like it. It does seem a little silly to hear about Laos referred to as “overseas” in the local press, because that’s one place you certainly don’t cross a sea to get to from here.