Spectroscopists associated transitions involving energy levels with
different 
values with different
groups of lines in the line spectra of the alkali metals. The line groups were
called sharp, principal, diffuse, and fundamental.
When the angular momentum quantum number was used to describe and explain these groups
of lines, s became an abbreviation for = 0, p meant = 1,
d meant = 2, and f meant = 3.
For consistency, higher values of the angular momentum quantum numbers are
designated alphabetically (g means = 4, h means = 5, and so on).