What is monograph and digraph in keyboard optimization problem?
The article uses monograph to refer to a single key, say "e", in a position on the keyboard. In the interest of efficiency, you'd want the frequently-used letters to be in locations where a model typist would have to expend the least motion to strike. A digraph, on the other hand, would be two keys which would be struck in sequence, like the "in" which I typed a few characters ago. In this case, we would be looking for a keyboard layout which minimized the effort depending not just on the absolute positions of the characters, but on the relative positions of the pairs.