Why copyleft matters to a first-time founder
A copyleft licence can require a founder who distributes modified or combined software to provide source code under the same licence. That can conflict with plans to keep a product proprietary or license it exclusively. The exact duty depends on the licence, how code is linked or combined, and whether software is distributed or only run as a service.
Examples of copyleft
A startup modifies a library under the GNU General Public License and ships it inside a desktop product to 5,000 customers. Distribution may require the startup to provide corresponding source code and the same licence rights for covered work. Replacing the library before launch may be cheaper than redesigning licensing after customers receive it.
Common misunderstanding
Copyleft is not the opposite of copyright and does not place software in the public domain. It uses copyright licences to grant permissions on conditions that preserve freedoms for recipients. Not every open-source licence is copyleft; permissive licences often allow covered code to be used in proprietary products with fewer sharing obligations.
When to use it
Use the term when reviewing open-source dependencies, contributions, distribution plans, or acquisition diligence. Record every dependency and its exact licence, then get specialist advice where obligations are unclear. Do not assume a repository marked “public” is licensed, or copy code without complying with its copyright and licence terms.