An important licensing and business strategy in commercial Open Source Software industry.
"With a dual-licensing approach, the company is protected by a GPL (or similar) license, because both competitors and potential customers who wish to embed/link with the GPL software must also GPL their own code. Since most competitors/customers don’t wish to do so, they are willing instead to pay for a commercial license. This simple yet subtle point is at the heart of the success of nearly every commercial open source organization." (http://robertogaloppini.net/2007/06/02/open-source-firms-enterprisedb-business-model/)