Electric field lines are a typical representation for the electric field concept. University students often misinterpret electric field lines, which may cause them difficulties understanding the electric field. The objective of this study is to explore how introductory electricity and magnetism instructors interpret electric field lines. We interviewed four instructors about several electrostatic concepts, to interpret a diagram and to discuss the electric field line representation. We focus on instructors' interpretation of the diagram, which consisted of the electric field lines of a quadrupole without displaying charges. We asked instructors to interpret the diagram and to compare the magnitude of the electric field at two positions. All four instructors tried to identify the sources of electric field as their first approach, while their answers varied for the comparison of magnitudes. Instructors' interpretations of electric field lines may elucidate why students often misinterpret this representation of the electric field.