There are a select few open source applications out there that provide both a free, completely open source version and a "pro" version that uses both open and closed source code. You just have to be careful to use the right license so you don't violate anything along the way.

Some open source applications have opted for a mixed-source strategy to better protect any closed source concepts that they wish to implement into an open source project. Some companies feel strongly that this better protects their positioning in the marketplace, despite the fact that closed source applications copy each other all the time. To be completely fair, there are some instances where combing the two licensing schemes can work and not cause any kind of community backlash. But as a general rule, dual licensing is best suited for either server applications or that of actual hardware appliances. Either the servers themselves, or even appliances such as household DVRs.

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