I still wonder whether there are two factors involved here: experience (from practice); and some sort of innate ability to be able to 'visualise' sounds and 'translate' them into a tune or chords.

It seems that some people are excellent musicians, but they are unable to play by ear (i.e. without having learnt from the printed music); or unable to listen to a piece of music and instinctively know how to play the appropriate chords; or whatever. I know a few like this myself.

I've been surprised a few times in the past when other musicians have seemed amazed (or at least fascinated) by the fact that people like me can (usually, there are exceptions!) pick up a tune and play along with it, or accompany a song or tune we've never played before.

I suppose I take this for granted, and it may be that I've got better at it with experience, but now I think about it, I think I've always been able to do this to some extent. I had piano lessons for a while as a kid, but before that I was able to play tunes and even simple chords on it without any musical training or knowledge. To me, being able to work out chords to a song or tune is no big deal - but I'm talking here about fairly simple music. (Ask me exactly how Nic Jones plays Canadee-i-o and I wouldn't be able to tell you. But give me enough time - lock me in a room with a guitar, the CD, a CD player, and no distractions - and I could probably work it out though.)

Maybe some people just lack this ability (to instinctively 'understand' how music 'works', perhaps?), and no amount of practice will make it happen for them? Some of the responses so far seem to me to indicate that might be the case. Does anyone else think this too, or am I barking up the wrong tree?