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Tetrads consist of four congruent regions which share boundaries with each of the three others.
This site discusses a recreational geometric puzzle. It is not a scientific publication.
After the introductory questions 1 to 3, the questions 4 to 8 are more autobiographical, and relate my own tetrad story.
In the other chapters several old and new tetrads are discussed, mainly tetrads made of congruent polygons with certain properties.
Also the construction principles of complicated tetrads are explained.
A heptagon tetrad with a quadrilateral hole.
The correlations between the angles are noteworthy.