It is then that the student realizes the first oscilloscope has its "coupling" control set to AC, while the second oscilloscope was set to DC. Now the student is really confused! The signal is obviously AC, as it oscillates above and below the centerline of the screen, but yet the "DC" setting appears to give the most accurate results: a true-to-form square wave. How would you explain what is happening to this student, and also describe the appropriate uses of the ÄC" and "DC" coupling settings so he or she knows better how to use it in the future?