The tone system consist of 12 different tones that are repeated again and again. There is a semitone between all 12 tones. 7 natural tones are used, and 5 accidentals.
The natural tones consist of the first 7 letters of the alphabet: A, B, C D E F G.
Below is a summary of the fretboard with all the natural tones. The upper horizontal line is the thinnest string on the bass. This string is called the G-string, as it produces the tone G when it is played without pressing a fret down.
If you put the finger on the 2nd fret on the G string, the tone A is generated, and so on. From the 12th fret the tones repeat themself. This means that the 12th fret on the G-string is also a G, 14th fret is an A, and so on.
Notice that the notes B and C and E and F is located right next to each other. Among the remaining natural notes there is a note with an accidental. These are referred to by a # or b. You can raise a tone (brighten it) by adding a #, and you can lower a tone (make it deeper) by adding a b. The tone lying between C and D (e.g., the 4th fret on the A-string) can thus be named by raising the C with a half tone so that it labeled C# or by lowering the D a half tone, so it is called Db.
Here is a list with names of all the 12 tones: