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CUSIP Check Digits Are Determined by the First Eight Characters

Article Quant Q&A · Author: PBD10017
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Summary

The document explains the role of the final character in a nine-character CUSIP identifier. That character is a check digit computed from the preceding eight characters by a deterministic rule. Consequently, two identifiers that share the same first eight characters cannot have different valid check digits under that rule.

The response answers the question directly and points to the check-digit algorithm as the basis for the conclusion. It does not describe the calculation procedure in detail, discuss identifier assignment or exceptions, or offer a broader security-selection method. Its practical lesson is limited but useful when validating or interpreting CUSIP identifiers: the final character is not an independent identifier choice once the initial eight characters are fixed.

Key ideas

  • A CUSIP consists of eight identifying characters followed by a check digit.
  • The check digit is calculated deterministically from the preceding characters.
  • Valid CUSIPs with identical first eight characters therefore share the same final digit.

Tags

Equities
Full text
# Is it possible for two securities to have the same first 8 characters of a cusip, but differ in the check sum?


# Is it possible for two securities to have the same first 8 characters of a cusip, but differ in the check sum?












CUSIP is a 9 character long identifier. The last digit is a check sum checking the first 8 previous characters. This seems to me that it is not possible for two securities to have the same 8 characters and different check sum. I know this is not exactly quantitative, but I haven't found a forum where I could ask.

Source: https://www.cusip.com/cusip/index.htm

## Answer by omikron (score 2, accepted)

https://quant.stackexchange.com/a/16391

No, 9th character is computed using deterministic algorithm described here: http://en.wikipedia.org/wiki/CUSIP#Check_digit_pseudocode.

Shown in full with attribution under the source's licence. Licence: CC BY-SA 4.0 (Stack Exchange)

This summary was written by Stratmill's research agent from the original; it is not a copy of the source.