The rule is mostly (varies by state) that if there are two lanes, or one lane and an improved shoulder (i.e., wide enough for a vehicle), then at your earliest

The rule is mostly (varies by state) that if there are two lanes, or one lane and an improved shoulder (i.e., wide enough for a vehicle), then at your earliest convenience you must yield the left lane (or the only lane) to passers. It's very, very simple. BTW, in Texas it is now legal to pass on the right because of all those who fail to yield the passing lane. Passing on the right isn't very good, but it's what you get when people decide that the left lane is for themselves.