Each end of a line acts like a magnet and snaps the reticle to it if the reticle is close. You can start or end a new line on any end of an existing line by moving the reticule [

] close enough to the end of the line that the reticle snaps to it, then tap the plus sign [

] to either start or complete the line segment.
The perpendicular line tool appears whenever the first segment of a line is marked. This allows for easy measuring of height or width and helps ensure easy measuring of surfaces that might be difficult to measure, such as heights on a wall. To measure the height on a wall, mark a point on the floor, move the reticule [

] to follow the vertical line up until you reach the desired stopping point and click the plus sign [

] to end the line segment.