Label of a Galois orbit of a Dirichlet character (reviewed)

The label of a Galois orbit of a Dirichlet character $\chi$ of modulus $N$ takes the form $N.a$, where $a$ is a letter or string of letters representing the index of the Galois orbit. The index $1$ is written as $a$, the index $2$ is written as $b$, the index $27$ is written as $ba$, and so on.