What is the stroke called that is used to create a serif in Celtic letters?

The correct answer, a fillet, refers specifically to the small, decorative stroke that helps define serifs in Celtic lettering. In the context of calligraphy, serifs are the projecting features at the ends of a stroke within a letter. The use of fillets enhances the visual appeal and can also influence the readability and distinctiveness of the letters.

Fillets in Celtic calligraphy are often characterized by their fine and elegant nature, contributing to the intricate designs that are a hallmark of the style. This decorative element not only serves a functional purpose in guiding the viewer's eye along the text but also serves as a signature flourish that identifies the style of Celtic calligraphy.

In contrast, the other choices refer to different elements of letterform construction: tails are typically long, curved strokes that extend from a letter, arms are extensions that connect two points of a letter without fully closing a shape, and nibs are the pointed ends of a pen or tool used for writing that dictate the width of lines. These terms do not pertain specifically to the creation of serifs in Celtic letters, making fillet the most appropriate choice in this context.