The path lines are the lines or trajectory of different fluid particles traversed by them at different points of time. Path lines are the outcome of Lagrangian method in describing fluid flow.
Let P1, P2 and P3 are the different fluid particle in a fluid flow. The paths traversed by them are shown in Fig. 4.2.
One path can intersect the other.
The path traced by a single particle of smoke coming out from a cigarette is a path line.
Note: For steady flow the path lines are identical to stream lines.