In my house growing up, the living room was formal, the den was where the family hung out watching TV and playing games.
For what it’s worth, I agree with Munch – in my home and those of people I knew, as well as on The Brady Bunch, the den was the father’s retreat. He might use it as an office, a hobby room, or a place to lie on a battered couch and watch TV. While the wife and kids might be allowed in occasionally, the den was definitely not a “family room”.
and I suppose if we had a question on what kind of grass a house has in the backyard, you would answer “astroturf”.
in the house in which I grew up:
the ‘den’ was dad’s office
the “studio” room was the place to hang out, play games, watch TV, etc., with company
the living room was for music, because that’s where the piano, the organ, my sister’s guitar, and my trombone were. Jams sessions ensued.
the dinette part of the kitchen was where we spent most of our time.
I cracked the top 10! Holy shnikes! throws a small party for self, eats cake