Ask HN: Show Code with Notes Alongside

Tell me about projects that allow code to be displayed on the left and notes about that code to be displayed on the right ?

I've seen this in the past but I can't find an implementation now.

Thanks.

EDIT: I should have said that the 'code on the left, notes on right thing' was overly prescriptive, as long as the notes and code are in columns and the notes sit alongside the relevant part of the code that's good as far as I'm concerned.