What is an ISIN (International Securities Identification Number)?

Key facts

How is an ISIN structured?

Its structure is simple to read once the parts are separated. The first two letters are a country code, the next nine characters are a national security identifier and the last character is a check digit calculated from the other 11.

Apple’s ISIN, US0378331005, starts with the country code US and ends with the check digit 5, and algoseek’s US Equities Security Master File (US1027) stores the full code in ISIN:

What does an ISIN look like in algoseek data?

That same code travels with the security into the market data. Daily products such as Primary Exchange Daily OHLC (US1013) and Trade and Quote Daily Bar (US1038) carry an ISIN column, described in the catalog as the “ISIN as of this trade date”.

The column also appears in the Detailed Adjustment Factors (US1005) and both IPO datasets (US1020, US1021):

With one shared code on each file, a price series, its adjustment factors and a third-party fundamentals feed can be matched without relying on a ticker string that may since have been reused.

The daily column is marked optional, though, so some rows arrive blank and a join needs a fallback for them.

Why does an ISIN change?

Where the ISIN gets harder to use is over long stretches of time, because the code follows the security’s registration and a corporate event that changes the registration can retire one ISIN and issue another.

The Security Master keeps this history in ISIN and ISINStartToEndDate as semicolon-separated lists, with an end date of 29991231 meaning no end date has been assigned. SecId 33008, whose tickers have included AA, ARNC and HWM, has carried four ISINs:

ISIN and ticker change on separate schedules, so neither one alone follows a stock through its whole history.

When is an ISIN not enough?

Venue is the other gap. A stock that trades on NASDAQ, on other exchanges and through the FINRA trade reporting facilities has one ISIN across all of them. PrimaryExchange names the exchange where it is listed, not where each trade took place, so venue-level work needs the exchange field on each trade and quote in the TAQ products.

Given both limits, algoseek tracks a security’s history with two identifiers of its own. The SecId usually survives ticker and name changes, but a restructuring can create a new one, and the ASID from the ASID Lookup (US1028) links those SecIds into one continuous security. ISIN remains the bridge to outside data, and joins on it work best date by date, matching each row’s trade date to the ISIN in force on that day.