Block Diagrams and the Extension of Timelike Two-Surfaces

The work of Finkelstein, Kruskal, Graves and Brill, Carter, and Boyer and Lindquist on the extension and schematic representation of 2-dimensional metrics is systematized and generalized. As a result, a number of extensions may be found by inspection. Some well-known examples are given, and the technique is applied to the ``soluzioni oblique'' of Levi-Civita.