A Ruby textual annotation is an annotation, typically short, applied to a letter, word, phrase, or name that appears in narrative text. Ruby annotations can indicate, for example, pronunciation advice, notes for translation, semantic annotations, etc. Ruby is used with Japanese, for example, in Furigana, which typically uses Hiragana annotations on Kanji base text. Ruby is also used in Chinese Pīnyīn (a phonetic system for transcribing the sound of Chinese characters into Latin script) as well as for some Korean names. In display or print, the characters of a Ruby annotation are frequently placed above the characters they modify, in parenthesis after the characters they modify, or to the right of vertically set text.

In BITS, as in NISO JATS, Ruby annotations are not used within any of the metadata elements such as <book-meta>, <book-part-meta>, and <sec-meta>.

The <ruby> element is an inline wrapper element that contains some of the document narrative text (inside a Ruby Base <rb> element) and associates that base text with a Ruby Textual Annotation (inside an <rt> element).