What are Secondary Mediants / Submediants?
I'm learning about secondary dominants and noticed references to secondary mediants and secondary submediants in a resource (Wikipedia). However, I haven't found any other references to them in internet searches. I think I could guess at the function of these types of chords but the lack of information leaves me wondering if they really exist in the lexicon of music theory. I also wonder if there isn't a different and more practical explanation for whatever function secondary mediants and submediants might play. Can anyone offer any clarity here?