Stars are born in gas clouds, dense and cold, which are mainly found in the disks of spiral galaxies. The distribution of young stars within the galactic disks provides us with information about the intrinsic properties of the parent galaxies, and has led us to develop different mathematical tools for their analysis. In this way, a discipline has been born, which we could call star formation geometry, which links the processes of star formation with the spatial structure, at different scales, of the galaxy. The appearance of a galaxy is not just a beautiful and fascinating spectacle, but a still photograph of what its star population looks like and how it is born.