Zoom B2FOUR Operation Manual page 55

Using flat tunings
All strings can be tuned flat from an ordinary tuning by 1 (♭× 1 ), 2 (♭× 2 ) or 3 (♭× 3 ) semitones.
Turn to set flat tuning.
NOTE:
Flat tuning cannot be used when the tuner type is CHROMATIC.
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