The guitar has to be tempered the same way that a piano does, and, while the placement of frets does this when you are playing notes on any given string, you have to make sure that the other open strings are tuned to tempered pitches, as well and not "perfect", the way violins are tuned--Most players intuitively re-tune fretted notes so that the pitch is right, but the open notes, and especially the open chords will be way out of tune--