Understanding Onion Architeture

I am trying to grasp the important concepts of Onion Architecture and had a question I was asking myself after reading an article. Look at the Domain in the architecture showed in this image: http://tonysneed.files.wordpress.com/2011/10/onion-proj.jpg?w=282&h=494

What is the practicality of separating the Domain.Entities and Domain.Interfaces in two separate projects instead of having one Domain project with a entities and Interfaces folder? I am not very experienced but I don't see a scenario where one would thank god he has the domain entities and the domain interfaces separated..

Solution

How about so that the interface definitions can be published to clients, as open-source, while the implementation remains proprietary. Just for one of many reasons why this is desirable.

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