When a company’s shares are listed on more than one stock exchange for the purpose of adding liquidity to the shares and allowing investors greater choice in where they can buy or trade their shares.

HSBC has its shares listed on the London Stock Exchange (LSE), The Hong Kong Stock Exchange, The Paris Stock Exchange and the New York Stock Exchange.
Standard Chartered has its shares listed on the London Stock Exchange (LSE), the Hong Kong Stock Exchange and The Bombay Stock Exchange.
BHP Biliton is listed on the London Stock Exchange (LSE) and the Australian Securities Exchange (ASX)
Shell has its shares listed on the London Stock Exchange (LSE) and the Amsterdam Euronext and the New York Stock Exchange (NYSE)
Unilever has its shares listed on the London Stock Exchange (LSE) and the Amsterdam Euronext.
Shell as a more complex capital structure, with Shell being made up of 2 classes of shares Shell A (class A) and Shell B (class B). Class A shares and Class B ordinary shares have identical rights, except related to the dividend access mechanism, which applies only to the Class B ordinary shares. This is due to different tax mechanisms on dividends in the UK, USA and The Netherlands.