You can build closed and proprietary software on top of GPL software (for certain definitions of &quot;on top of&quot;), as long as any changes to the GPLed software are released as source, and LGPLed software is only linked to dynamically.<p>Example: If I wrote an alternative GUI system to replace X and the open window managers, I wouldn&#x27;t necessarily be forced to open-source that system.<p>Real-world example: My employer uses Linux in some of its products, integrated with our proprietary software in license-compliant ways (and we&#x27;ve got a legal office that verifies that we&#x27;re following the licenses for <i>all</i> the software that we use, to avoid legal trouble).