A serial number that contains only 2 unique digits, for example "19919191".

Example of Binary serie:

(Photograph credits: Mercadolibre: billetes.antiguos. This piece also is Radar serial number, 6 digits of-a-kind serial number and Bookend serial number (2 digits))

A banknote which a sequence of digits is the same at both ends, for example 15723157. For illustration about the amount, the base is an serial interval 00000001-90000000 with 90,000,000 pieces. The most known category are:

• 2-digits bookend: 899,999 pieces (2 digits), for example 15757815.
  
• 3-digits bookend: 89,999 pieces (3 digits), for example 85623856.
  
• 4-digits bookend (also a 4-digits Repeater serial number): 8999 pieces (4 digits), for example 56875687.
  

Example of Bookend serial number (2 digits):

(Photograph credits: Mercadolibre: billetes.antiguos. This piece also is Radar serial number, Binary serie and 6 digits of-a-kind serial number) Example of Bookend serial number (3 digits):

(Photograph credits: Mercadolibre: billetes.antiguos. This piece also is Binary serie and 7 digits of-a-kind serial number) Example of Bookend serial number (4 digits):

(Photograph credits: Mercadolibre: billetes.antiguos. This piece also is Binary serie and Doubles serial number)

It's a group of 4 banknote bundles, which are 4.000 banknotes.

It's a group of 10 banknote straps, which are 1.000 banknotes