A banknote which a sequence of digits is the same at both ends, for example
157231
57. 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.