a janus phrase

I hit on a Janus term just now.

There are Janus *words*, or contronyms:   words that can mean either of two opposite things.   Think of "cleave," which can mean cutting something apart or joining things together.   Or "sanction," which can mean reward or punishment, approval or disapproval. 

I'm sure there are several Janus terms that aren't single words.    Just today, I was pulling into our driveway, which marks the beginning of a very shady several-yard of our neighborhood, and thinking that trees are often a class marker:  worse neighborhoods are often treeless and sit in merciless sun, whereas tonier neighborhoods are often described as "leafy," and block off lots and lots of direct sunlight.

That is, shady neighborhoods aren't shady neighborhoods, but shady neighborhoods are not shady neighborhoods.