Short answers, pulled from the story.
The FTSE 100 Index is the United Kingdom's best known stock market index, tracking the 100 most highly capitalised companies listed on the London Stock Exchange. It is also known as the FTSE 100, the FTSE, or informally the Footsie.
The FTSE 100 Index started on the 3rd of January 1984 at a base level of 1,000 points. It replaced the Financial Times' own FT 30 as the market's headline index after its public unveiling on the 14th of February 1984.
The FTSE 100 Index is maintained by FTSE Russell, a wholly owned subsidiary of the London Stock Exchange Group. FTSE Russell originated as a joint venture between the Financial Times and the London Stock Exchange, and the index is calculated in real time and published every second the market is open.
The FTSE 100's worst single-day return came during Black Monday in 1987, a drop of 12.22%. Over the two days of that crash the index fell 21.73% in total, while futures trading volume hit a record 9,111 contracts in a single day.
The FTSE 100 reached its all-time closing high of 10,910.55 points and its all-time intraday high of 10,934.94 points on the 27th of February 2026. The index had started at a base level of 1,000 points on the 3rd of January 1984.
The FTSE 100 Index weights companies by free-float capitalisation, so restricted stock such as shares held by insiders is excluded from the calculation. FTSE Russell uses a formula developed by Hermann Paasche, chained continuously as a Chained Paasche Index, to adjust the index's divisor around corporate actions and constituent changes.
The FTSE 100 is considered a weak indicator of the UK economy because many of its constituent companies are internationally focused. This makes the index sensitive to swings in the pound sterling's exchange rate, and the FTSE 250 Index is considered a better read on the domestic economy.