Originally Posted by mind_drummer
I think it's more than just that - I read the Jim Riley Modern Drummer article about the Nashville Number, he explain pretty much in details.
BTW, here's Jim "Lips of an Angel" charted with the Nashville Numbers
And you will also see parentheses used to indicate a single bar which has more than one chord rather than being underlined. Any number by itself is a single bar and if a bar is 2/4 or anything other than what the original time signature is, it will be notated above the bar with "II" for example for a 2/4 bar. There really aren't any standard rules on this type of chart except for the numbers which make it simple to change the key of a song on the fly.