Celtic letters should be 5 to 6 pen widths tall based on the height-to-width ratio of the text-block and the width of the reed pen nib. Calligraphers used interlines to determine the heights of different parts of the letters.
The height of Celtic letters should be 5 to 6 pen widths tall. This is derived from the height-to-width ratio of the text-block in the manuscript and the width of the nib of the reed pen used by calligraphers.
Each line of text was further divided into interlines, which helped determine the heights of various parts of individual letters.
Although there is no ruling on the parchment, scribes might have used horizontal guidelines on a board to help maintain consistent proportions in their writing.
Learn more about Celtic Letters