The open strings on a 4-string bass, from thickest to thinnest, are called E – A
– D – G.
Each string is represented as a note on the staff:
In music we talk about the distance between notes as whole steps and half steps.
A half step on the bass is the distance from one fret to the next. A whole step
is the distance of two frets. All the natural notes are separated by a whole step,
except for B to C, and E to F, which are separated by a half step.
Here is a diagram of the open strings and the first set of natural notes: