why sud nse or bse default? contracts are exchanged between buyers and sellers in f and o its the clients who take all the risk. exchange is just an organisation to manage this bazar. so i think it is broker and clients who can default but they cud be one or two at a time. for a while even we suppose that nse defaults and if u have shares of a particular company that company will pay u dividend bcose that share is actually ownership of that company though govt has to find another medium to do so.