The following is a reprint of data sheet #45 from the Guild of American Luthiers. They don't know who wrote it and it never made it into publication. But it's so incredibly useful that it's one of their most requested articles (you can download a PDF version from them here). To anyone who ever gets frustrated when tuning their guitar, this article explains why and I strongly advise you to read it. I promise you that accepting the information presenting here will make your guitar and your music sound better and your life much easier.