Dovetailing
Definition. Dovetailing turns a two-dimensional infinite grid into a single list by sweeping a finite quantity (the short diagonals, or the length), so that every cell is reached after finitely many finite stages and no infinite row can block the walk.
It lists the positive rationals and the finite binary strings (Lecture 2, Examples 3 and 5), and in Unit 2 it runs infinitely many computations "in parallel" by interleaving their steps. See Countable.