Check digit

An ISIN is twelve characters long, and every one of those characters has a job. The very last one has a special job: catching mistakes.

That last character is called the check digit. It is not chosen at random. It is calculated from the eleven characters that come before it, using a fixed formula called modulus 10 "Double-Add-Double".

If even one character earlier in the ISIN is typed wrong, the check digit calculated from the correct ISIN will not match. That mismatch tells the system something is wrong.

An ISIN has three parts: a 2-letter country prefix under ISO 3166 (IN for India), a 9-character alphanumeric basic number, and a single check digit at the end — 2 plus 9 plus 1 makes the full 12-character code.

The check digit is computed according to the modulus 10 "Double-Add-Double" method set out in the ISIN Standard, ISO 6166. Applying this formula to the first 11 characters produces exactly one digit, which becomes the 12th character. Anyone, or any system, that already knows the formula can recompute the check digit from the first 11 characters and confirm it matches the 12th — establishing that the ISIN is valid.

Take the illustrative ISIN INE475C01012: IN is the country prefix, E is the third-position company-type indicator, 475C is the company serial number, the next 01 denotes the instrument type (equity here), the following 01 is the issue number, and the final 2 is the check digit computed from everything before it.

A DP keying in a client's ISIN for a demat request types INE475C01013 instead of the correct INE475C01012 — a single-digit typing error in the very last character.

Because the check digit is computed from the first 11 characters by the modulus 10 Double-Add-Double formula, the depository system can immediately recompute what the 12th character should be — 2, not 3 — and reject the mismatched ISIN before the request is even processed, rather than letting a demat request for 500 shares worth Rs 3,20,000 proceed against the wrong security altogether.

Had the typing error instead occurred in an earlier character, say the company serial number, the resulting recomputed check digit would again fail to match, catching the error just as reliably.

Chapter 7, section 7.2 (International Securities Identification Number), breaks the 12-character ISIN into its three components and names the check digit as the last of them, worked through with the ICICI-style illustrative example the workbook itself uses.

Expect a question on how many characters an ISIN has (12), what the three components are (2-letter prefix, 9-character basic number, 1 check digit), and what method computes the check digit (modulus 10 Double-Add-Double).

• The check digit is the 12th character, not a separate number appended to the ISIN. It is part of the 12-character code itself.
• It is computed by modulus 10 "Double-Add-Double", a specific named method under ISO 6166 — not just any checksum.
• The check digit validates the whole ISIN, not just itself — a single wrong character anywhere earlier in the code will cause the check digit to fail to match.
• Do not confuse the check digit's position (12th, last) with the third-position company-type indicator (E, F, A, B or 9), which is a completely different single character earlier in the code.