Today, banknotes are printed with a combination of different processes going from intaglio over offset and letterpress to gravure. Forging of banknotes is rather difficult if the note is well designed. See here an example based on the details of the (outdated) Belgian 1000 BEF note (click the pink letters) and some details of the (new) 50 Euro (same letters, but not clickable). For more details over the Euro go down to the link "The Euro"
A) You have the well known watermark, in this case a portrait and the signature of the painter Constant Permeke. Seen normally, his head is nearly invisible, but his signature dark. Viewed against the light, it's opposite !
On dark surface____________________________________________against the ight
B) Nearly every better banknote is printed on a special paper that reacts under ultraviolet light :
back side viewed under UV
C) Another method can be the use of magnetic inks, but a lot of forgers found that out and if you know how, these inks are not so difficult to be "home made" and I even don't know if these inks are used on the Belgian 1000 BEF note, but I know for sure that all greenback dollars are for a part printed with them .
D) The metallic stripe: Inbedded in such a way in the paper that when viewed with reflecting light invisible on the front side and only partially visible on the back side, but holding the note against a light source you will see a continous stripe. And looking with a magnifying glass one can read "1000 F" over the whole length.
Back side___________________________View through
E) Precision printing: The image of the Permeke museum in the lower right corner of the front side is perfectly in register with the image in the lower left corner of the back side. Holding the note against the light, the following will be seen:
F) Microprinting: Some parts of the note are printed in such a fine lining that they can not be seen with a human eye. Under a magnifying glass it looks like that:
G) Hidden image: Due to a special printing process, when looking straight on the surface of the banknote, the block "G" (left to the 1000 on the front side) looks just like a pepper and salt grey, but hold under a certain angle, you can read the digits 1000; when holding the note as landscape, the 1000 appears in dark on a light background, when holding as portrait, in light on a dark background.
H) The relief mark for the blind : The horizontal stripe is for the factor: one stripe means times one, two stripes times two and three stripes times five. The vertical stripes stand for the digits: I stripe for hundred, II for thousand and III for ten thousand. So, theoretically, a sign like =III means 20000.
I) Copy protection: Some parts of the note are printed in a way that they look just grey, but on copying them, they will change their pattern into black and white lines
The grey cloud over the sailboat.
J) OVI printing: The 1000 on the back side is printed with a special "OVI" ink that changes color under different viewing angles (like pearls)
90 degrees______________________about 130 degrees
Another good site is the homepage of
The Bank of England