If you are playing along with another instrument, it is essential that your mandolin be in tune with that instrument. Tune the open strings to the corresponding notes of the accompanying instrument. E.g., To tune to a piano, tune the open 4th string to the G note on the piano, as shown on the keyboard diagram. Then tune your mandolin to itself from this note, or tune each string of your mandolin to those notes of the piano shown on the keyboard diagram.