US currency is printed in sheets of 32 bills organized in an 8 × 4 arrangement. The last two digits of the serial numbers are the same for all 32 bills on the sheet. One hundred of these sheets are stacked, cut to size by a guillotine, then bundled.

The newer versions of US currency have two letters in front of the serial number and one behind it.

Some collectors specialize in collecting radar notes, notes with many 7‘s in the serial number, or notes with the same serial numbers from different Federal Reserve Banks.

In the year 1999, the US government began issuing paper money that was less likely to be counterfeited. The bigger picture is one of the reasons. There is a hologram behind the picture.