# what does the .<...> notation mean?

I was reading about Basic Rings, and the notation

`ratpoly.<t> = PolynomialRing(QQ)`

popped up. What is the <> for? It's not a function call, although I can see what it does. And the ratpoly has no meaning. I substituted skunkpoly with the same result. I can't seem to find much about it.

See also https://ask.sagemath.org/question/50560/