A plane described by the X and Z axes. A convention used to describe an [[acoustic]] [[wave train]] display in which the amplitude of the [[signal]] voltage is used to modulate the intensity of a photographic beam of light to produce varying photographic density on the [[Z axis]], and time in microseconds on the X axis. See [[intensity modulated time|intensity modulated-time]] and [[Micro Seismogram|Micro-Seismogram]].