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.