A phrase used to describe the architecture of modern effect units that can truly process a stereo input as two discrete channels. When early digital reverbs and multi-effect processors hit the market they had processors in them that could generate a stereo output (often reverb) from a mono input. In fact, that’s the only way they worked. The processors inside only had one input, and the output was a synthesized stereo signal. Some of these devices later showed up with stereo inputs to the unit, but those signals were summed to mono before entering the processor itself. If you were connecting the unit in line with a stereo signal the dual inputs provided a way to keep the dry (unaffected) portion of your signal in stereo. As the cost of processing technology dropped manufacturers were able to include dual processors in equipment. One of these processors could be dedicated to each input allowing two different effects to be used at once (one on each input). These processors could often have their control signals ganged together so they would act as one true stereo processor. Nowadays there are processors on the market that can accept two signals directly and process them.