The representation of magnetic fields by magnetic field lines is very useful in visualizing the strength and direction of the magnetic field. Each of the magnetic field lines forms a closed loop. The field lines emerge from the north pole (N), loop around to the south pole (S), and continue through the bar magnet back to the north pole.

Magnetic field lines follow several hard-and-fast rules:

1. The direction of the magnetic field is tangent to the field line at any point in space. A small compass will point in the direction of the field line.
2. The strength of the field is proportional to the closeness of the lines. It is proportional to the number of lines per unit area perpendicular to the lines, also called the areal density.
3. Magnetic field lines can never intersect each other, which implies that the field is unique at any point in space.
4. Magnetic field lines are continuous, forming closed loops without a beginning or an end. They are directed from the north pole to the south pole because the north and south poles cannot be separated.

They are distinctly different from electric field lines, which generally begin on positive charges and end on negative charges or at infinity. If isolated magnetic charges or magnetic monopoles existed, then magnetic field lines would begin and end on them.

Magnetic field lines are defined to have the direction in which a small compass points when placed at a location in the field. The strength of the field is proportional to the closeness or density of the lines.