no if you have a classical guitar you'll screw it up ( i dunno bout acoustic and electric guitars)
cuz of the wood or something i dont remember exactly. that's what my teacher said.
just buy a tuner.
standard tuning = E A D G B E
drop D tuning = D A D G B E
both are common tunings... make note of how similair they are, and get a cromatic tuner, without it it is hard to tune precisely

also when tuning your guitar, make sure you pull the strings (literally) out of tune and retune to whatever tuning you are are in so all the notes are tuned correctly...
You can get by if your guitar is out of tune (as in not at concert pitch), but it's not advisable - it means you can say bye bye to playing along with anything. It also means oyu won't begin to learn to recognise notes, because every time you play you'll be tuned differently.
Actually called Mark!

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