Auctioning is one of the most transparent methods for allocating ETS units and puts into practice the principle that the polluter should pay for their emissions. It is a complex system, but the premise is really quite simple: a proportion of units consistent with the cap and the number of free allowances available is allocated for auction. These are then sold on a specific date to emitters through a single round, sealed-bid auction, which then provides for an initial price of a unit. Once allocated this way, the buying and selling of units between emitters and other intermediaries can then take place on what is known as the secondary market.