A matrix scheme (also known as a matrix sale or site, and as a hellevator, excavator or ladder scheme) is a business model involving the exchange of money for a certain product with a side bonus of being added to a waiting list for a product of greater value than the amount given.[1] Matrix schemes are also sometimes considered similar to Ponzi or pyramid schemes.[2] They have been called "unsustainable" by the United Kingdom's Office of Fair Trading.[1] A matrix scheme is also an example of an 'exploding queue' in queueing theory.