Logical square. (german)
<a href="//commons.wikimedia.org/wiki/User:Sebastian_Wallroth" title="User:Sebastian Wallroth">Sebastian Wallroth</a>
2009-02-12
Own work by uploader. Based on <a href="https://de.wikipedia.org/wiki/File:Logisches_Quadrat_korr.png" class="extiw" title="de:File:Logisches Quadrat korr.png">de:File:Logisches Quadrat korr.png</a>