The FTSE 100 tracks the 100 largest companies listed on the London Stock Exchange by market capitalization.
The index was launched on 3 January 1984 and is maintained by FTSE Russell, an index provider associated with the London Stock Exchange Group. Its name is commonly pronounced “Footsie.”
The FTSE 100 is weighted by free-float market capitalization, so larger eligible companies have greater influence on its value. It is reviewed quarterly, helping the membership reflect changes in company size and eligibility. The index includes many internationally active businesses, meaning its performance is not a pure measure of the United Kingdom economy.
A common mix-up is the FTSE 250. That index covers the next 250 eligible companies below the FTSE 100 in size, although the boundary can change as companies move between indexes. The FTSE All-Share is broader and includes most eligible companies listed on the London Stock Exchange.