I've just been reminded of a thing I've been wondering for a while, which it occurs to me that my readers might be able to help me with.

There are a couple of pairs of words or phrases which are very similar in meaning, but which I vaguely remember having been told, at some point during my childhood, that there was a clear distinction between:

• ‘Recall’ and ‘recollect’: I remember somebody telling me these described specific and different ways to pull something out of your memory, but web-searching and dictionaries now suggest to me that they are in fact simply synonyms.
• ‘Polar opposites’ and ‘diametric opposites’: I remember being told that these too were distinct concepts in some specific way, but as far as I can now tell they're both just superlative forms of ‘opposite’ indicating that things are as opposite as they can possibly be.

It's mildly frustrating me that I can't remember what the distinction was supposed to be in either case. I don't much mind whether the distinctions turn out to be real or not (except insofar as their reality would have a bearing on how easy they were to look up), but I would like to know what they might have been.

So I don't suppose anyone else here might believe in a well-defined distinction between either of these pairs of words, and therefore be able to enlighten me as to what either of those distinctions might be? It wouldn't guarantee that they were the same distinctions which I've lost down the back of my brain, of course, but even if not they might jog my memory a bit.

(This is also reminding me, now I come to think about it, of a wordplay-oriented radio panel game I remember listening to as a child, whose actual name I've forgotten but one of whose rounds I remember being called ‘Deft Definitions’. In this round panellists were given a pair of nearly synonymous, or sometimes nearly homophonous, words and were asked to pithily define them both in a single sentence so as to illustrate how they differed. However, unlike the definitions I'm after here, Deft Definitions was humorous in intent: the answers often deliberately missed the real point, because it was funnier that way.)