A directed line segment is a line segment with a direction and distance. Unlike a vector, however, a directed line segment has a unique start and end point. The starting point, also called the initial point, can be identified by following the direction of the line segment. When determining the position of a point that partitions the line segment into a specific ratio, the direction of the line segment is important to know.

As shown on the line, point $P$ partitions the directed line segment $\overline{AB}$ into a $1:2$ ratio. That implies $A$ is the starting point where one part is followed by two parts of three parts total. Conversely, the same point partitions the directed line segment $\overline{BA}$ into a $2:1$ ratio. In that case, the starting point is $B$ where two parts are followed by one part of three parts total.