Electric field lines are imaginary curves whose tangent at any point gives the direction of E at that point. Key properties: (1) Lines originate on positive charges and terminate on negative charges; (2) Lines never cross — if they did, the field would have two directions at the intersection; (3) The density of lines (number per unit area perpendicular to lines) is proportional to |E|; (4) Lines are perpendicular to equipotential surfaces; (5) In a uniform field, lines are parallel and equispaced; (6) Lines contract along their length (attraction between unlike charges) and expand laterally (repulsion between like charges). For a dipole, lines emerge from +q, curve through space, and end on -q.