What is a serif in typography?

A serif in typography refers specifically to the decorative stroke or feature that extends from the ends of the letters' main strokes. These strokes often have a specific style, either angular or rounded, depending on the typeface, providing a visual enhancement and aiding in the readability of the text. Serifs are commonly seen in traditional typefaces such as Times New Roman, where they help guide the reader's eye along the lines of text, creating a more fluid reading experience.

The other options refer to different concepts within typography and calligraphy. A straight line used in lettering is not specific to serifs and could pertain to various aspects of letter design or basic line work. A type of paper used for calligraphy is unrelated to the definition of serifs, focusing instead on the material aspect of calligraphy practice. Finally, a method of spacing letters relates to letter positioning (kerning and tracking) rather than the embellishments found on the letters themselves. Thus, "a decorative stroke at an angle on letters" accurately captures the essence of what a serif is in typography.