What are basis?  it’s the price difference between the price of an energy in one market and the price of an energy commodity in a completely different market.

Locational basis is the “different” market can be at a different location 

Calendar basis risk, or calendar spread risk, is the risk that arises from hedging with a contract that doesn’t expire on the same date as the underlying exposure.

As an example, a large consumer of gasoline might decide to hedge their exposure to gasoline price by purchasing NYMEX RBOB gasoline futures. In this example, the consumer is exposed to calendar basis risk as NYMEX gasoline futures expire on the last day of the month prior to the delivery month i.e. the October 2013 RBOB gasoline futures contract expired on September 30, 2013.  While many might assume that this consumer has no choice but accept the basis risk.  There are other instruments which will allow them to mitigate their exposure to calendar basis risk.

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http://www.mercatusenergy.com/blog/bid/38368/An-Overview-of-Energy-Basis-Basis-Risk-and-Basis-Hedging