The South Africa Top 40 Index (SA40) is a capitalization-weighted stock index that tracks the performance of the 40 largest companies listed on the Johannesburg Stock Exchange (giving the index its second name of JSE Top 40 Index). JSE is the largest stock exchange in Africa. Though the number of companies within the index is relatively small, together they represent over 80% of the JSE listed companies' market cap. As a result, the JSE Top 40 can give investors a pretty good idea of what's happening on the South African stock market.