The series of degrees forms the line. The direct line is the series of degrees between persons who descend one from another. The collateral line is the series of degrees between persons who do not descend one from another, but who descend from a common ancestor.
In the direct line, the number of degrees is equal to the number of generations between the heir and the deceased. In the collateral line, the number of degrees is equal to the number of generations between the heir and the common ancestor, plus the number of generations between the common ancestor and the deceased.