ISIN, CUSIP, SEDOL and Tickers

Buy one share of a large multinational company and that single security can legitimately answer to four different codes at once, depending on who's asking. A US settlement system wants its CUSIP. A European counterparty wants the ISIN. A UK back office might still reference the SEDOL. And the trader on the desk just calls it by its ticker. None of these are wrong — each identifier was built to solve a different problem, at a different point in the settlement chain.

The four identifiers

Identifier	Scope	Format	Built for
CUSIP	US and Canada	9 characters, alphanumeric	US clearing and settlement (pre-dates ISIN, still the domestic standard)
SEDOL	UK (historically), still used globally	7 characters, alphanumeric	Originally the London Stock Exchange's own identifier
ISIN	Global	12 characters: 2-letter country code + 9-character national number (often the CUSIP or SEDOL) + check digit	Cross-border trading and settlement, standardised by ISO 6166
Ticker	Exchange-specific	Short alphabetic code, e.g. AAPL	Human-readable trading and quoting, not a settlement identifier

The ISIN is not really a fifth, independent code — it's built by wrapping the existing national identifier (a CUSIP for a US security, a SEDOL for a legacy UK one) inside a country prefix and check digit. A US stock's ISIN literally embeds its CUSIP: US + the 9-character CUSIP + a check digit.

Why the ticker is the least reliable of the four

A ticker is convenient exactly because it's short and memorable, and unreliable for exactly the same reason — tickers get reused. A company that delists frees its ticker up for a completely unrelated company to claim later, exchanges reassign tickers after mergers and spin-offs, and the same ticker string can mean different companies on different exchanges entirely (a ticker collision between a US listing and an unrelated foreign listing is routine). A database keyed on ticker alone will silently mismerge history across unrelated companies the moment a ticker gets recycled. This is exactly the failure mode covered in Ticker Changes, CUSIP/ISIN Mapping and Symbology.

CUSIP, SEDOL and ISIN are far more stable, but not permanently fixed either — a corporate action like a merger, a reincorporation, or a change of listing venue can trigger a genuinely new identifier for what an investor experiences as "the same company," which is why corporate action data feeds need to explicitly map old identifiers to new ones (see The Corporate Action Event-Type Taxonomy).

A worked example

Apple Inc.'s US-listed common stock carries CUSIP 037833100 and ISIN US0378331005 — literally US plus that same CUSIP plus a check digit 5. Its ticker on Nasdaq is AAPL. A European fund buying the same shares through a different clearing chain would still reference ISIN US0378331005; a UK back-office system built before ISINs were universal might still carry a legacy SEDOL entry for it internally, mapped alongside the ISIN.