The speed of a line- I learned about the speed of the line during one of my first critiques classes at NCSA. It amazed me that it was possible to tell whether a drawn line was made quickly or slowly or with a combination of speeds.
Recently on an episode of Billions you may have seen some great calligraphy. Even now the invitation was used in the scene to denote wealth and tradition. How was it made. Was it copperplate, engraving?
Often calligraphers are challenged to write multiple calligraphic hands on one paper. Sometimes the challenge is to get large cracked letters. A rough paper that causes skips and cracked edges of letters with a larger broad edged pen will most likely not work for a pointed pen of any size. That is when knowing paper making history and Islamic techniques comes in handy.