The pool of master limited partnerships with an investment grade rating is pretty small. In fact, only 16 of them have been rated BBB- or higher by Standard and Poor's. That means there are plenty of MLPs like Northern Tier Energy (UNKNOWN:NTI.DL), Linn Energy (NASDAQOTH:LINEQ), or Alliance Resource Partners (NASDAQ:ARLP) that are out of luck. In this video, contributor Aimee Duffy looks at the MLP space and explains why the aforementioned MLPs may never see an investment grade rating and what that means for the investors who hold them.

Fool contributor Aimee Duffy has no position in any stocks mentioned. Fool contributor Tyler Crowe owns shares of LINN Energy. The Motley Fool recommends Alliance Resource Partners. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.