A banknote which a one digits from the serial number repeats many times, most important about 1 to 3 digits less than the length of the serial number. If all digits of the serial number is the same, it's called a Solid serial number.There are two type: in-a-row and of-a-kind. For illustration of amount of pieces, the base is an serial number interval 00000001-90000000 with 90,000,000 pieces.

A.- in-a-row
All repeated digits are next to each other. The amount of pieces are:

• 5 digits in a row: 26,244 pieces, for example "12222258"
  
• 6 digits in a row: 2187 pieces, for example "57777779"
  
• 7 digits in a row: 163 pieces, for example "85555555"
  

Example of 5 digits in-a-row serial number:

(This piece also is Binary serie) Example of 6 digits in-a-row serial number:

(This piece also is Ternary serial number) Example of 7 digits in-a-row serial number:

(This piece also is Binary serie)
B.- of-a-kind
All repeated digits is split by one or more distinct digits. The amount of pieces are:

• 5 of a kind: 367,416 pieces, for example "27229322"
  
• 6 of a kind: 20,412 pieces, for example "41446444"
  
• 7 of a kind: 649 pieces, for example "33363333"

Example of 5 digits of-a-kind serial number:
Example of 6 digits of-a-kind serial number:

(Photograph credits: Mercadolibre: billetes.antiguos. This piece also is Radar serial number, Binary serie and Bookend serial number (2 digits)) Example of 7 digits of-a-kind serial number:

(Photograph credits: Mercadolibre: billetes.antiguos. This piece also is Binary serie and Bookend serial number (3 digits))