 Versions of this process can be carried out with other mathematical entities as well. Projective geometry adds additional points ‘at infinity’ to the Euclidean plane, one for each family of parallel lines, to help explain features of visual geometry, like the way that parallel railroad tracks appear to meet at the horizon, infinitely far away. Riemannian geometry extends the complex numbers by adding just a single number \(\infty\) that one can approach simultaneously ‘in all directions’ in the complex plane. These alternate geometries provide foundations for material discussed in section 2.2, and also   section 8.2. Several of these are discussed in the entry on   nineteenth century geometry,  and others are discussed in topology textbooks under the topic of ‘compactification’.