The starting line is an invisible line between a buoy marking one end of the line and a pole with an orange flag on the Race Committee boat.  Since you can’t see this invisible line, it may be difficult to tell exactly where it is unless you are near an end of the line. The line is usually set perpendicular (square) to the direction of the wind so there is no advantage to starting at a particular end of the line. If there is a significant change in wind direction, the start may be postponed to square the starting line to the new wind direction.