Electrically insulating material so foldable as to form an interphase insulator so configured as to be inserted in the slots of the stator or the rotor of an electric machine to prevent separate coils inserted in the same slot to be in direct contact with one another and to prevent the coil heads of different coils to be in direct contact is described herein. The interphase insulator may be so folded that the coil heads are properly insulated from one another.