Dear word sommelier: I notice that in your note on chesterfield you used sofa and couch interchangeably. Isn’t there a difference?
Of course there’s a difference. Sofa has two syllables and four letters and is soft, with just two voiceless fricatives at the front of the mouth and a round vowel (“ohhhh” like a sound you might make sinking into a plush sofa), and it has overtones of sol-fa and soda and perhaps Sufi and suffer and so far and shofar and maybe even loofah… Couch, on the other hand, has one less syllable and one more letter, but just three phonemes – of which two are compound: a diphthong and an affricate. It is a harder word, to be sure, with its voiceless stop onset and its voiceless affricate ending. But somehow that doesn’t seem to bother the people who sit on couches – not even the echo of ouch or the taste of crunch and catch and cow and crutch…
Words are, as I have said many times, known by the company they keep. But these two words actually have very similar collocations – here’s what wordandphrase.info gives for them: sofa: adj asleep, comfortable, sectional, plush, living-room, Victorian, sagging, convertible; noun chair, room, table, living, cushion, leather, bed, back, arm, pillow; verb sit, lie, sleep, watch, lay, fall, lean, settle, seat, face; couch: adj asleep, comfortable, living-room, comfy, overstuffed, lumpy, sagging, upholstered; noun room, potato, chair, living, leather, back, arm, cushion, TV, bed; verb sit, sleep, lie, lay, fall, settle, lean, seat, face, sprawl.
Mind you, they do keep some different company socially. You can’t draw a nice isogloss for them – a line on a map that shows roughly where people stop using one word and start using the other, like with soda and pop (and, in the southern US, generic coke). The divider, inasmuch as there is one, is more one of social set. People whose lives are softer seem to like sofa better. If you see the play Six Degrees of Separation by John Guare (or watch the movie made from it, starring Will Smith, Donald Sutherland, and Stockard Channing), you will see a scene in which a rich kid who went to prep schools is teaching a poor black kid how to fit into the rich social set, and one point he makes is that the piece of furniture is not a couch, it’s a sofa.
Not that it’s as tidy as that. Many people use both, depending on context or whim, even if they prefer one to the other (usually couch to sofa). If they want to emphasize the softness or sound somehow more high-class, they might go with sofa. Or they might associate one word or another with some particular person or social set from their own lives. Each word is couched differently for each person.
Oh, yes, couch has a verb, too, which sofa does not. In fact, it has a much wider set of uses, noun and verb. It shows up in many contexts where something is lying or set together or into another thing. Which is reasonable enough, given its etymology. You very likely know that French se coucher means “go to bed” or “lie down”. A couch is not simply something you sit on; it’s something you can lie on – like the psychoanalyst’s couch, not sofa. So you see that there are these extra little collocations that show up farther back.
Oh, yeah: that’s where the French coucher comes from, by the way. Latin collocare. Meaning “put in its proper place” or “lay with” (not “lie with” – that comes with the French).
And sofa? It comes from Arabic. You may have seen, in real life or in pictures, an Arabic room furnished with a low platform on which is laid a carpet or carpets and various cushions for sitting or reclining on. That is what Arabic ṣoffah refers to, that platform and its cushy furnishings.
And the different denotations? Don’t they refer to different things? Well, the analyst’s couch has only a half back and one arm (or head end), and that matches a particular type that can be distinguished from a sofa with its full back and arms. But in actual common usage, especially in North America, one simply can’t draw a neat distinction, not even a fuzzy-bordered distinction like between cup and mug. You just have to go by feel and whim. You know what effect you would get by saying sofa potato instead of couch potato; go from there.