Letters not only make visual sense. Letters make handwriting sense. Visual sense is a consequence, an effect of the rational, logical and systematic use of tools, materials and methods. The particulars, though, are embodied and can’t be explained with words, they need to be experienced.
In the Wikipedia article about skill, we read: ‘Hard skills, also called technical skills, are any skills relating to a specific task or situation. It involves both understanding and proficiency in such specific activity that involves methods, processes, procedures, or techniques. These skills are easily quantifiable unlike soft skills, which are related to one’s personality.’
Actually the opposite seems to be true: high technical skill is not measurable.
Technical skill evolves through embodied learning that consists of creative iteration. This also applies to the handwriting sense. To understand the handwriting sense we need to create letters by hand: handwriting sense needs to be experienced rather than described, quantified or explained.
Jan Tschichold wrote ‘Nothing is better for the understanding of the construction of letterforms than the following handwriting practice with the broad nibbed pen…Real knowledge is only gained by diligent practice. Theoretical considerations are totally useless.’ (Schriftkunde, Schreibübungen und Skizzieren, ein kleines Lehrbuch der Schrift für Setzer und Graphiker, 1941)