An electronic tuner will display a note regardless of the octave, so an A on a guitar at the open 5th string, will read the same as an A at the 2nd fret of the G string. You can tune a mandolin with a guitar tuner. Set your tuner to auto rather than manual, and simply read the notes G, D, A and E and don't use the B. The A on a mandolin is an octave plus one step higher than a G on a guitar, however, just to eliminate confusion, within the same octave, A is higher than G. The G on a mandolin is the same pitch as the G on a guitar, but all the other notes are either one or two octaves higher than the same named note on a guitar.
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