They do both. For example, compare the ETFs "United States Oil Fund" (ticker USO) with "United States 12 Month Oil Fund" (ticker USL). Every month, USO buys the

They do both. For example, compare the ETFs "United States Oil Fund" (ticker USO) with "United States 12 Month Oil Fund" (ticker USL). Every month, USO buys the next month's future as the previous month's future expires. Also every month, USL buys the next year's future as the future bought a year previous expires. You can look at their charts here: http://bit.ly/ar6Kxm http://bit.ly/ar6Kxm