# Marked Bills and how to track them

You always hear about marked bills in movies. How do you mark a bill and then track it?

I know you can tell where it was passed, but how do bankers know what to look for? I have always wondered about this.

I always assumed that they used invisible ink that fluoresces under ultraviolet light.

Another material that has been used to catch criminals in Gentian Violet. In its powder form, it can be dusted over the item that is likely to be stolen. Anyone who handles the object will get the powder on their hands. When the powder comes into contact with water, it produces a violet stain that is obvious and difficult to remove.

Yeah, but your average teller isn’t going to be checking bills with a UV light. They have to know what to look for, something very specific.

sometimes marked/unmarked bills is just a matter of serial numbers. if you get a bunch of bills numbers 41453000000 to 41453999999 then you just need to send out a paper telling that range and tellers can take action if someone comes in with a large chunk in between. unmarked bills would be just randomly numbered ones where there is no simple range to look for.

What about the machines used to count currency? Can they be programmed to look for specific serial numbers, or are they just looking at the denomination?

Counting machines I’ve used merely riffled the bills, then counted how many there were. If you mixed denominations, it wouldn’t know or care - it would tell you how many bills was in the stack.

Are there machines that sort and count bills automatically? I’ve used coin sorting machines, but I’ve never had enough bills in one place at one time to need a sorter/counter.

Without going into to much detail, there’s two sorts of these machines. The high-speed counters that banks use don’t identify denominations at all. They only count pices of paper, which the operator must tell the machine the value of. The other sort is a quite a bit slower, and uses a technique identical to the bill acceptors in vending machines. In these, a beam of light is shined through the bill and a photosensor measures the amount of light that passes through. Each denomination of bill creates a unique pattern of rising and falling light levels, and the machine is programmed to tell which pattern corresponds to which denomination, if any.

Yep.