"Partner songs" are songs that can be played and sung simultaneously, because they have the same chord pattern. Examples are:

"Bill Bailey" and "Just Because"

or Irving Berlin's:
"Play A Simple Melody" and "I Want to Listen To Rag"
and
"I Wonder Why" and "You're Not Sick, You're Just In Love"

I recently discovered, while listening to the radio in my car and vocally experimenting, that if you sing "Cottonfields" slowly, as Belafonte does, "Hard Times Come Again No More" can be sung with it as a partner song.

Then, of course, there is Simon & Garfunkel's "Scarborough Fair" and the "Canticle" they sing along with it. And at Christmas time we hear a neat reggae version of "Mary's Boy Child" with "Oh, My Lord" superimposed on it. And while the Kenny Rogers Christmas song "Pretty Little Baby Child" is playing, you can sing "Sukiyaki!"

Anyway, I'm interested in what sets of partner songs other folks know. They're a lot of fun to sing and can really sound great.

BTW, if there is another term for this phenomenon, please let me know. I tried to see if there were any previous threads on the subject, but doing a search under "partner" yielded nothing relevant.

Genie