KNOWOL gathers and shares knowledge.

In accordance with the spirit of a free and open internet, the text portion of this website is available under the Creative Commons Attribution-NonCommercial 3.0 United States license (CC BY-NC 3.0 US), or any later version. A summary of the terms of this license is available, as well as its detailed terms. All images with the KNOWOL watermark are licensed under the same terms.

Appropriate credit simply means a link back to the article the text or image was taken from. If you share our content on social media, please post a link to our website rather than downloading and posting the image directly.

Rights in the trademarks and service marks of KNOWOL, as well as the look and feel of this website, are not licensed under the terms mentioned above. However, we support you making full use of your rights under fair use, fair dealing and other exceptions and limitations to trademark.

KNOWOL is informative, not authoritative

KNOWOL is designed to inform, not to be the final, authoritative reference. We do our best to quote accurate sources, but if you are looking to reuse or redistribute something posted on our website, please check the licensing and source of the thing itself (for example, on the individual web page, or the link provided to the original source) after you’ve looked at this document.

If you want to use an image found on this website – read the license terms associated with that image. If you do not explicitly see a Creative Commons license, err on the side of caution and do not use the image. KNOWOL is not responsible for the incorrect usage of any material on this website.

In case of a conflict between this document and other documents (for example, this document and the private page of a photographer) the other document should take precedence. If you discover such a conflict, or if your copyrighted work is featured on this website and you would like it removed, please email