An electricity market is a system for effecting the purchase an sale of electricity using supply and demand to set the price. Wholesale transactions in electricity are typically cleared and settled by the grid operator or a special-purpose independent entity charged exclusively with that function. Markets for certain related commodities required by (and paid for by) various grid operators to ensure reliability, such as spinning reserve, operating reserves, and installed capacity, are also typically managed by the grid operator. In addition, for most major grids there are markets for electricity derivatives, such as electricity futures and options, which are actively traded. These markets developed as a result of the deregulation of electric power systems around the world. This process has often gone on in parallel with the deregulation of natural gas markets.