Contour Lines: The Lost World

Contour lines totally fix the structure of leftcontinuous t-norms. For each t-norm, these contour lines are determined by the corresponding residual implicator. Most properties involving residual implicators can now easily be translated into properties involving contour lines. As the portation law expresses associativity, we dispose of a powerful tool for constructing left-continuous t-norms. In particular, we focus on the decomposition and construction of t-norms that are rotation invariant w.r.t. one of their contour lines.