The D55A is a double-beam instrument, with two identical timebases, MAM and "B". A versatile switching arrangement enables either timebase to be coupled to either beam, or alternatively enables both beams to be coupled to one timebase. Alternatively, timebase "B,T can be switched to trigger timebase MA" . The point at which timebase "A" is triggered can be varied by a delay pick-off control, calibrated from 0-100. Thus the instrument has a unique facility whereby a signal displayed on the lower beam has a bright-up marker superimposed on it. This marker may be moved over the entire length of the trace by the delaying sweep control; the portion of the signal covered by the marker is determined by the setting of the main sweep and is simultaneously displayed across the entire length of the upper trace. A special 5" Spiral PDA C.R. tube, with a double-gun assembly, ensures that both traces are completely independent. This enables both a completed waveform and a selected and magnified part of it to be displayed simultaneously. The double-gun assembly also eliminates the need for "chopper circuits", which are normally used when the phase relationship between waveforms is being examined. In addition, the instrument is ideally suitable for use as a general purpose oscilloscope where a large display is required.