A run of straight segments through as many points as you click.
The Polyline draws a run of straight segments through every point you click and leaves it open. It
traces something that turns: a route, a run of pipe, a flow through a detail.
Draw it: click once at each turn, then press Enter. The run ends at the last point placed,
open, with no segment back to the start.
Keys while placing
Enter: finish the run.
Backspace: remove the last point.
Escape: also finishes, with the points placed so far. It does not discard the run.
Start ending / End ending: the same nine styles, both bare by default. That is what makes a polyline usable as a routing arrow: click the path, then put a head on the end.
Notes
The head sits on the final segment and follows its direction, so the last click decides where the arrow points.
Double-clicking does not close or finish the run; it just adds a duplicate point.
See also
A polyline never closes. Where the run comes back to its own start and the enclosed area is the
point, use Polygon, which closes and can be filled.