This thesis is an important contribution of technical knowledge to the limited literature on calligraphic expertise. The main objective of this work is to demonstrate that handwriting and handwritten signature are important elements of security in those documents containing one or the other. To that end, the origins of handwritten communication are sought, describing the scriptural tools used in its graphical exteriorization and the intrinsic characteristics that they give to the writing. The importance of the fact that the shapes of the letters are stored in the human brain, which is the main technical foundation that determines the graphic individuality, is highlighted. A detailed and novel study of the characteristics of the writing rhythm and the stroke is made, as they are integrator elements of all the graphical features of the individual elements. Graphic aspects which reveal the scriptural rhythm and stroke, so they can be taken into account in the comparative analysis, are determined. This provides an increased security and reliability in the conclusions of the expert reports on forensic handwriting expertise, thereby writing and handwritten signature are presented as important security elements of documents.