There are many OER available under different licenses. It’s the same level of difficulty to recognize and legally use compatible materials
This is true that OER published under particular Creative Commons licence may not be compatible with each other but traditional copyright is not straightforward either and may lead to even more questions than compatibility of CC licences itself.
The first step is to understand the conditions and licensing rules. In fact, they are clearly described so a user can see how the work is available for reuse. Then it is highly unlikely that the user will face immediate legal action for violating the terms of an OER license.
There are some ways to help acting in line with those rules. Those who do best to understand the license, can use tools providing support in differentiating their compatibility such as Compatibility Chart developed by Creative Commons.
If the user is still uncertain whether found materials can be merged or not, one solution is to use only materials published under the same licence. Although, it limits the range of available content, the user gains assurance of acting legally. Most OER databases’ suppliers provide a possibility to publish the content on various conditions. Thus, OER are labelled properly according to the licence chosen by its author. OER services and even general search engines, like Google, offer filtering options by type of use or even exact licence. This is possible due to “three-layer” design of CC licences that contains: a traditional legal tool (understandable for lawyers), a “human readable” version of the license (a format understandable by ordinary users) and the last layer – a “machine readable” version that provides summary of the key freedoms and obligations written into a format which is recognized by software systems, search engines, and other kinds of technology. This approach to CC licence design guarantees OER to be searched easily and filtered by the type of licence. The user is able to search the content published under compatible licences in order to remix them.
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