Hi Zhong
Some of the answers here are a little off. A guitar is hardly ever in tune and like a piano, the guitar has a tempered-tuning which means that you have to tune it in octave here and there to get an acceptable tuning in open position and at the 12th fret.
A guitarist can compensate easily as he is playing by bending a note slightly or whatever to get it in tune. When you are playing, it is hardly noticable but to yourself.
The only time your guitar sound perfectly in tune is when you put brand new strings on it. The next time, it is slightly off. As long as you can play well, its not tht important. At my studio, half of my recordings are done with a half-tuned guitar and its up to the guitar player to adjust to it.
When I see these so-call guitarist tuning and tuning and tuning the guitar is mind-boggling. You are there waiting to see something amazing come out after all that effort only to see a G chord or en E open.
edit...Brad...nice post. Have to remember that one LOL.
edit.... Any good Guitar player, even with a broken string on stage (happened to me) will camouflage for that broken string if he knows his scales on a fingerboard.
Krist sake, I even broke a 4th string and nobody noticed. If you practiced string-skipping, broken strings are never an issue.
Secondly, when you shred and do lots of bending in between riffs, the guitar is never perfectly in tune. I would like to see a guitarist pick only one sour note while I shred on 1/32 or 1/64th notes. Good luck.
Simply put, the guitar is easy to compensate no matter what. Obviously, I am not implying that a guitar that is completely out of tune is the norm. But like the song Desafindao >> "Slightly out of tune"