A copyleft license allows you to alter and distribute Open Source code under the condition that your modifications are just as open to the public as its source. The transparency fosters a sense of collaboration among software developers and compels them to share their improvements on the Open Source software. Although these measures reinforce the concept of free software, the obligation makes copyleft licenses more stringent than permissive licenses. Therefore, copyleft is a subset of Open Source software since it allows software developers to use free source code. Copyleft-licensed software is not ideal for proprietary software, but you can still commercialize it.