Page "Foundations of mathematics" Paragraph 39

Later in the 19th century, the German mathematician Bernhard Riemann developed Elliptic geometry, another non-Euclidean geometry where no parallel can be found and the sum of angles in a triangle is more than 180 °.

It was proved consistent by defining point to mean a pair of antipodal points on a fixed sphere and line to mean a great circle on the sphere.

At that time, the main method for proving the consistency of a set of axioms was to provide a model for it.