The serif is the finishing stroke, without which, this capital “I” would be just a rectangle. I decided to make the serif larger on the top left and bottom right, reflecting the way a scribe would use an edged pen, sliding the nib on its edge, to get ink primed on the whole surface to start the stroke, and cutting off cleanly at the end. I started with a 45 ° angle on the bottom right, and half that ( 22.5°) on the bottom left. I then made equal steps to transition between this angle and the vertical. This type of construction results in a parabola. The diagram on the left shows the steps in shades of gray. I then changed the angles into a smooth curve and flipped the whole structure to make the top of the I.