What does pen width measure?

Pen width specifically refers to the measurement of the thickness of the strokes made by a pen or brush as it applies ink to the surface. This concept is critically important in calligraphy, as it influences the overall proportion and visual impact of the letters being created. Proportion in calligraphy encompasses the relationship between different elements of letterforms, which can often be affected by the pen’s width; thicker strokes can give a bolder appearance and alter the balance between various parts of a letter.

The other options do not directly relate to what pen width measures. Height of letters pertains more to the vertical space each character occupies rather than the stroke thickness. Length of strokes is focused on how far a stroke extends rather than its thickness. Lastly, width of the writing surface is about the area on which the calligraphy is performed, which does not pertain to the characteristics of the pen's stroke. Thus, understanding pen width as an element of proportionality in letter forms is essential in achieving effective calligraphy.