What is monograph and digraph in keyboard optimization problem? I have been working on Bangla keyboard optimization. I've taken this paper as a reference. In this paper they have introduce two terms monograph and digraph. But it is not clear how they have calculate these two things. In this paper it is written - 


  A monograph is an isolated key which is struck in the process of typing a text. A digraph is a sequence of two consecutive keys hit in the process of typing a text. This idea allows the distinction between rules that concern the absolute position of a given key and the relative position of two consecutive keys.


What I am assuming that digraph is those two consecutive character that are used often in any language word. Like in English language ph, qu etc. But I can not understand what is monograph. 

Therefore I would need a clear explanation about monograph and digraph and what is the method to calculate these in respect to 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.