regarding cartridges? And whatever else those terms are related to.

From the first transducer (the cartridge) to the last (the speaker), both channels have to maintain the correct polarity to remain in phase. The left and right speakers have to move outward in unison when excited-mostly by bass notes that are common to both channels. If one of the speakers or if one side of the cartridge is wired backwards, the speaker will move inward with the common bass note and the other speaker will move outward. The bass note will be somewhat cancelled as the sound pressure created by one speaker will be reversed by the other. The result is hollow bass. Reverse one speaker wire and play a mono record. See what happens.

RCA out-phased one of their earlier stereo consoles at the cartridge. A matching accessory reversed the reversal but if you plugged it into anything else it sounded wrong. Sounds like marketing to me.

Anyhow, other than that one model series, far as I know nobody else ever did that so it is of little concern unless you are rebuilding that model.

I think that she actually IS consdiering buying one of these reverse-phased, units, and RCA SHC-4. Don't worry, Mary, it is really not a big deal to overcome the reverse phasing....there are several threads on here that discuss it in detail. Google the search term "antique radio forums reverse phasing rca" and you will see several threads about it in more detail.

All of RCA's early high end stereo models were out of phase. It was a very sophisticated design. RCA’s stereo cartridges, stereo tape players and mono tuners all had out of phase signals. They all use a 3-pin audio connecter so that standard in-phase signals can not be connected to them via standard RCA jacks. Depending on the model, the phase was corrected at the output transformer primary or secondary windings and combined in mono mode. There were some big advantages to this technique. It allowed the stereo amp to simulate push-pull output when running in mono for greater power output. There was a lot of common mode noise reduction. Turntable rumble and hum were significantly reduced. Radio broadcasts were still mono and most records played on these sets were mono so these sets ran in mono most of the time. These amps only have a single 6V6 output tube on each channel. Whether running with the amps in stereo or combined, they sound quite powerful for what they are.