C.R.O.
AC/DC mode switch to select the desired coupling: DC coupling (Direct Coupling) allows both DC and AC signals through. If the input has both A.C. and D.C. components, the output includes the DC bias voltage) and will be displayed with reference to the ground. If there is no A.C component (say 9 V battery) the output is a straight line of the applied DC voltage with ref. to ground
A.C coupling means that only A.C. signals are allowed to pass from the input terminal through to the measuring devices within the CRO. This is achieved by passing the input signal through a large capacitor which "blocks" any D.C. component of the incoming signal. Switch to the AC coupling for measuring only the oscillating input signal with reference to the ground with no DC voltage included in it.