The Cac-40 is the principal French stock index. It is a market-value weighted index covering 40 French stocks. The index is float adjusted which means that the market capitalization used to weight each share is calculated using the number of shares available for public trading not the total issued.

See also: http://www.euronext.com/trader/summarizedmarket/stocks-2634-EN-FR0003500008.html?selectedMep=1