Is Marginal Revenue not always equal to the price?

Marginal Revenue is equal to the price in perfect competition, but MR is also defined as the revenue obtained by selling one extra unit of the good, so how is it not always the case that MR=P ? Revenue from one unit of a good = Price of the good.

Or is that definition not true, namely MR should be seen as $$\frac{\partial R}{\partial Q}$$ (which deosn't really make sense for goods that are discrete things, but let's just admit it does?)

• "Marginal Revenue is equal to the price in perfect competition" So are there market structures other than perfect competition? If so, perhaps you should check if it holds there? – Giskard Nov 7 '18 at 8:41
• Yes that's my question, why wouldn't it always hold? – John Nov 7 '18 at 8:42
• And as I said, perhaps you should check another market structure. I am not sure what exactly you are asking about, perhaps it is about the concept of a firm being a "price taker". – Giskard Nov 7 '18 at 8:43
• Hmm I know that a firm being a price taker means it has no influence on the price but that's about it, and I don't know what's the link between having that property of being a price taker and the property MR=P – John Nov 7 '18 at 8:45
• Perhaps you should piece this together? If you don't see the difference between a price setting monopolist and price taking competitive firm, I don't know what your question is. – Giskard Nov 7 '18 at 8:47