Ruby is a type of annotation associated with a base text that is often used in Japanese, and to some extent in Chinese also, to provide pronunciation information for ideographs, and sometimes short explanations. The name 'ruby' originated from a named font size (about half the size of the normal 10 point font) used by British typesetters. In Japanese this is known as furigana.

The Ruby Annotation Recommendation describes how to mark up text so that it is clear which is the base text and which is the ruby annotation. (This is not a CSS specification.) There are simple and complex ruby models. This slide shows the simplest form of markup described in the Recommendation. The text in the <rt> element is the annotation. The base text is in the <rb> element.

A user agent that displays ruby will normally display simple ruby text above horizontal base text and in a smaller font. For vertical text the ruby would by default be displayed to the right.

For a more detailed description see Ruby Markup and Styling.