Trading contracts have different months.

usoilapr is the December trading contract of US crude oil futures, and usoilmay is the trading contract of US crude oil futures in May. A futures contract is an agreement in which a buyer agrees to receive an asset at a specific price after a specified period of time, and a seller agrees to deliver an asset at a specified price after a specified period of time. The price that both parties agree to use for future transactions is called the futures price. The specified date on which both parties must enter into a future transaction is called the settlement date or delivery date. The asset that both parties agree to exchange is called the “subject.” When an investor takes a position in the market by purchasing a futures contract (i.e. agreeing to buy at a future date), it is called a long position or going long on futures. On the contrary, if the position taken by the investor is to sell a futures contract (that is, to assume the contractual responsibility to sell in the future), it is called a short position or going short on futures.