The concept of allograph in graphonomics and handwriting recognition is concerned with the fact that in handwriting, one particular letter from an alphabet can be realized using a number of shapes. The most evident occurrence of an allograph is in the upper-case and lower-case shape for one letter (cf., a vs A). However, within a writing system, individual writers use their personalized and characteristic shape for a letter (Figure 1). This poses many problems in optical character recognition. As an example, Figure 2 shows many ways in which writers may give shape to the word optimum in the Latin alphabet.

Related: Allography.

Figure 1. Examples of allographs for the letter t and the letter k

Figure 2. Samples of the word optimum, written by different persons.
Regardless of trivial size and slant differences, it is the use of
different allographs which characterizes a writer's hand.