The words we use in America (or the USA, or just the US, if you insist) are in a deplorable state.  I'm old, and my days is almost done, as Enid Blyton would say, so my level of concern for these things is pretty low, but in the interest of exactness and precision, it makes sense to clarify the meaning of words, when there is ambiguity.

For decades, American have used the words couch and sofa interchangeably.  I think that they ought to mean different things.  But we could use them interchangeably, because using them that way doesn't really do a lot of harm.  I'm going to describe what I think should be how we use them, and my reasoning, and you can take it or leave it.

Many years ago, when affluent men had large, private offices in their homes, it was customary for them to have an upholstered Bench along one wall.  At one end was a section, about 18 inches, that was hinged, and could be raised, so that either the person whose office it was, or a patient, could take a nap on it, or even sleep on it overnight.  They were broader than the typical bench; not quite as wide as a single bed, but say three feet wide, and with a moderately soft cushion on which you could get comfortable.  These are couches.  You could, as I said, sleep on them comfortably.  Notably, couches would not have backs, but would have the raised--or raiseable--section at one end, sort of like a chaise.  A chaise would have that portion permanently raised, whereas a couch could be made completely flat if desired.

A sofa is quite a different thing; it is something like an armchair designed for three people, more or less.  Also, it would have a back, and arms.  Also, the seat surface need not be completely horizontal, but angled back like a bucket seat, in which case the seat back would also be slightly reclined.

What most families have in front of the TV is a Sofa.  What you have to lie on when you're being psychoanalyzed, is a couch.  Consider that if the seat of a sofa happened to be angled, it would not be comfortable to sleep on.  In contrast, a couch, being a glorified bench, would not have that problem, and indeed one of its functions is precisely to be available for sleeping on.

Well, if any of my readers have been accustomed to using these words in a different way, and find it difficult to conform to my suggested uses, I will not be offended if you do it your way.  I feel that if you have two words, and the distinction between them begins to fade, you have lost a word, and made two words stand for the same thing.  Sometimes this is progress; other times, this is the opposite.

Arch

P.S. I recognize that some illustrations would make this much clearer; I will attempt to persuade the family illustrator to provide some helpful artwork.