When I asked the question in the original posting, I now realise it was because I had a particular introduction to music (in the 'active participation' sense); I sang, tried and failed to play various instruments until I found one that let me (Anglo) and then used its pattern to become familiar with others using similar patterns, like harmonica and melodeon. Although I sang solo, communally and later in a formal group it was melody that got me started and harmonies that I developed into. Like others on the thread, I don't read (but will probably learn now that my daughters are so proficient) and couldn't name a chord to save my life.
But I have learned to recognise enough about them to try being helpful when a tune I'm playing changes key at some point that has an accompanying guitarist puzzled. I'm limited, in that I'll give them what I think is the root note and only occasionally guess at what version of minor chord it might be, but that seems satisfactory. Guitarists' ability to learn chords merely by watching has always intrigued me, but that's probably because of my 'melodic' introduction to learning. The responses to this thread have opened my eyes to other entry points.
Now that I've moved away from the bright lights the most frequent session I get to are at the major festivals and, because of my dance band background, many of the sessions are on dance tunes (non Australian influences are Irish, Scots, English & American mostly, with some Swedish, German, French & others sprinkled in). Age has wearied the memory cells so that I'll recognise the name but forget how it starts, until the first notes; then it and all the subsequent tunes in the whole bracket are coming out of the fingers. Or I'll be rattling away on tunes and someone will ask the name(s) and I become embarrassed because I can't remember. It's some consolation that others have the same experiences.
Which brings me to Guest Terry's comment. "I have no musical training at all, but (usually) find it easy to play the chords to folk tunes/songs that I've never heard before, particulary when I'm part of a band."
Terry, I reckon the experience you described is an excellent training. It might not be complete (as mine hasn't been) and it might not be formal (as mine wasn't, like many others around Mudcat) but. like mine, yours seems to have worked. It mightn't have worked as well for someone else but, them's the breaks. I use your comment because it gave me the easiest entry to address notions I've picked up, in several postings, about practice.
While agree with whoever it was who indicated that continuous repetition of errors (something like that) shouldn't be regarded as successful "Practice", it seems to me that every time you session with someone or even just noodle by yourself through something that intrigues you, that's Practice. And probably very effective.