http://www.financial-spread-betting.com/Spread-betting-indices.html What are the main weightings behind the FTSE 100 index. Chris Purdy, Sales Trader at Spreadex responds. The FTSE 100 index is by far the most popular spread betting market. We have hundreds and hundreds of trades every hour. People like to think of it as a diversified index but really when you look at the constituents by sector it is not as diverse as the S&P 500 or even some of the indices we see in continental europe that have fewer constituents.