If you have the low E string tuned to D and you tune the rest of the strings to it, you will get D Tuning or in other words your guitar will be tuned to D (one whole step down from standard)
Standard tuning looks like this
E A D G B e
So D Tuning is like this
D G C F A d
It just gives a slightly heavier sound to your music. Many artists (Children Of Bodom) use D tuning instead of standard tuning.
Similarly your guitar can also be tuned to C or any other note and you will get different notes on the other strings.
Note: D tuning is COMPLETELY different from Drop-D tuning. Drop-D only tunes down 1 string whereas D tuning requires tuning down of all the strings.
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