This leaves the air gaps (G), if any, between core sections. In the gaps, the magnetic field lines are no longer confined by the core. So they 'bulge' out beyond the outlines of the core before curving back to enter the next piece of core material, reducing the field strength in the gap. The bulges (B F) are called fringing fields. However, as long as the length of the gap is smaller than the cross-section dimensions of the core, the field in the gap will be approximately the same as in the core.