This idea for 'curves':
This idea for 'lines':
I am not quite sure what's happening here, but it's not the first time this has happened to me. One theory is that by having sketched out some ideas, subconsciously I can relax and in so doing become more receptive to other ideas - both the images above were obvious but I never noticed them until I tried to capture the ideas I had sketched out.
It's a bit like when you try to remember something but no matter how hard you try, you cannot. Then, later, when you are thinking about something completely different, the thing you were trying to remember just pops into your head.