Document Examination holds many facets and handwriting comparison analysis is a major one.
We write from left to right while going up and down. Handwriting is movement and movement becomes habitual.
Each author will have recognizable habits and is the reason why we can identify someone's writing.

The primary process is determining a unique combination of common characteristics and/or unique but uncommon characteristics in Known exemplars. This unique set should reappear in the Questioned exemplar for writing to be authentic. Significant deviation from a predetermined unique set of common characteristics suggests an author other than the author owning the unique set of common characteristics.