I have never put too much thought into licencing. Maybe because I have not created much or the fact that even the things I have made I have not tried to share thus no need for a licences ( I think).
That does not mean I know nothing about licences, I do. It’s just I have not had the need to consider all the aspects. So I did some reading. (mainly from wikipedia). One of the principles of Copyleft is Freedom, mainly these four - freedom to use the work, freedom to study the work, the freedom to copy and share it with other and the freedom to modify the work, and the freedom to distribute modified and therefore derivative work.
So in a nutshell copyleft means that the intelectual property can be freely distributed and modified with the requirement that the same rights are preserved in the work that has derived from that property.
There are different types of copyleft:
- Strong and weak copyleft - most popular/well-known free-software licence using strong copyleft is GNU General Public License.
- Full and partial copyleft - full means that all parts of the work (excluding the licence itself) can be modified and distributed under the terms of the work’s copyleft licence. Partial in contrast means some parts of the work are exempt from the copyleft, permitting some modifications under the terms other than the copyleft license
- Share-alike - this condition in some cases imposes the requirement that any freedom granted regarding the original work has to be granted on the exact same or ocmpatible terms in the derived work. Some Creative common licenses are examles of share-alike copyleft licences.
So to conclude in an ideal open source world free licenses that preserve the “freeness” in the derived works should be used :) Or just go with WTFPL :)