What is the relation between CUSIP and ISIN?

CUSIP (Committee on Uniform Security Identification Procedures) is an alphanumeric code that uniquely identifies a North American financial security.

ISIN (International Securities Identification Number) is an alphanumeric code that uniquely identifies a financial security globally for the purposes of facilitating clearing, reporting and settlement of trades.

For a North American financial security, its ISIN code is constructed based on its CUSIP code as follows:

ISIN #: US{CUSIP}C

where:
  US: 2-character country code representing North America. 
  CUSIP: 9-character CUSIP code representing the security.
  C: 1-digit checksum of preceding 8 characters. 

For example, the CUSIP code for the Common Stock of Apple Inc. is 037833100. Its ISIN code is US0378331005 and constructed as:

US 037833100 5
-- --------- - 
|  |         |- the checksum 
|  |----------- the CUSIP code
|-------------- the country code

⇒ Converting CUSIP to ISIN

⇐ CUSIP Checksum Algorithm

⇑ Introduction on CUSIP

⇑⇑ CUSIP - North American Financial Security ID