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

The term "fillet" accurately describes the stroke used to create a serif in Celtic letters. In calligraphy, a fillet refers to a thin line or stroke that connects a serif to the main body of a letter, often adding visual interest and complexity to the character's form. This is particularly significant in Celtic calligraphy, where elaborate and decorative serifs contribute to the unique aesthetic of the lettering.

In contrast, the other terms refer to different elements of letter construction. A "tail" generally signifies a decorative stroke that extends from a letter's stem, while an "arm" is a horizontal or slanted stroke that helps form letters but does not pertain specifically to serifs. A "nib," on the other hand, is the pointed end of a pen used to apply ink to the paper, rather than a stroke associated with letter structure. Thus, the specificity of "fillet" as a description for the stroke creating a serif makes it the correct choice in this context.