The Perpendicular Segment tool creates a segment that is perpendicular to another segment, real or imaginary. We say “real or imaginary” because really all you’re doing is select two points and then drawing a new segment perpendicular to the line passing through those points. If there happens to be another segment or a side of a rectangle or triangle lying along that line, then the new segment will be perpendicular to it too. There is only one construction option, that is, which end of the line segment will be used for Snapping. This is useful because your finger is at the “top” of the segment. In order to make the bottom of the segment snap to some point near the bottom, you’d have to use the “snap to bottom” option. Here’s an example of Perpendicular Segment construction: