Drop is a good indicator of how flat shooting a load is because it's independent of sight adjustments. The bullet starts to drop immediately after it leaves the gun's barrel. At any given range, drop is measured from the line of departure (LOD) to the bullet. The line of departure is the same as the center of the gun barrel's bore. Even though the term drop already implies that the bullet is always below the line of departure, by convention, drop values are shown as negative numbers.