Market capitalisation is calculated by multiplying the total number of outstanding shares with the current share price. However, some companies have a dual-listed corporate structure whereby the organisation is listed as a separate legal entity on multiple exchanges. Thus, market capitalisation is the sum of the product of the number of outstanding shares and the associated price, on each exchange.

For example, Rio Tinto has two separate legal entities - Rio Tinto PLC (listed on LSE) and Rio Tinto Ltd (listed on ASX). Their most recent annual report lists the number of outstanding shares as 1,252.1 million and 371.0 million for PLC and Ltd respectively. As such, Rio Tinto's market capitalisation is 239 billion AUD (quoted accurately on Google) instead of 272 billion AUD as quoted on Yahoo Finance.