What is the rationale in 2019 for serial numbers on US currency?

They can still be tracked down to who is holding them at the moment

@SEKA but who is doing the tracking? Who or what agency is copying down serial numbers? I know what can possibly happen, I am wondering if it is actually being done.

That’s classified. If I tell you I’ll have to kill you

One reason is to deter counterfeiters. Identical serial numbers reveal a bill as fake, even if otherwise a very good copy. To replicate on fake currency would require an additional printing.

@zenvelo I agree, but to detect duplicates you have to be tracking the numbers in the first place. That’s my point – who is tracking the numbers on a day by day basis?

If I used a $10 bill AM939878588M at the gas station today, and you use a duplicate AM939878588M at Target tonight – how is it determined that it’s a duplicate?

Most of our currency has a metal strip that records the serial number & the bank keeps a record of any duplicates.It’s all computer generated & NO person is sitting in the back room recording the SN’s.

@elbanditoroso But if you get stopped by police and you have three bills all with the same serial #, busted!

@elbanditoroso “If I used a $10 bill AM939878588M at the gas station today, and you use a duplicate AM939878588M at Target tonight – how is it determined that it’s a duplicate?”

That’s unlikely to be detected. But a person passing around multiple bills with the same SN will be spotted fairly quick.

I worked in a Bank and part of the job was to keep a note of the serial numbers start and end of a bundle that was placed into a automatic counter then noted on the bundles before placed into the vaults.
The letters signify the series.

It’s not harder to machine-read a serial number printed in a font that is desinged to be machine-readable than it is to read a barcode or QR code.
The Treasury collect age data about the bills to understand durability. The age can determine in some cases whether a bill will get cleaned up and sent back out or get destroyed. There are probably lots of other things tracked to help normal flows of paper bills between regions etc.

The numbers are tracked to a certain extent as mentioned above. Money sent to banks are recorded, but at any given moment it’s not like the fed knows that you just spent a bill with a particular serial number on it to buy your McDonald’s hamburger. Unsophisticated counterfeiters might make hundreds of bills with the same serial code, but I bet they are better than that these days. I had a bill in my till once when I was worked at Bloomingdale’s that didn’t say In God We Trust. When I called security they couldn’t understand why that mattered. Idiot.

Another good point made above is keeping track of how much cash is in circulation, and how long a bill stays in circulation. Paper bills have a fairly short life.