Sensex, short for Sensitive Index, is the Bombay Stock Exchange's benchmark of 30 large and financially sound companies, weighted by free-float market capitalisation. Started in 1986 with a base value of 100 for 1978-79, it is the index most Indians hear on the news. Its constituents overlap heavily with Nifty 50, so the two indices move together almost all the time; Sensex simply has fewer stocks and a larger number. A level of 80,000 on Sensex and 24,000 on Nifty describe the same market. You can invest in Sensex through index funds and ETFs, and BSE offers Sensex futures and options, which have grown in volume in recent years. For a beginner, the useful habit is to watch the percentage change rather than the point change: a 400-point fall on Sensex at 80,000 is only half a percent.