Your guitar tuner might have two or three modes, which are Guitar mode, Bass mode and Chromatic mode. If you want to tune your guitar to the standard EADGBE tone, you should use the guitar mode. If you want to drop your D tune to DADGBE, you should use Chromatic mode since lower D is not a standard tuning for the guitar. The chromatic mode also tells you what notes you’re playing on your guitar and how accurate or out of tune they are. This also helps you better understand your guitar tone and adjust the string for a low or high tone.