Unfortunately, this ruby doesn't refer to a glamourous woman. This more prosaic technical term refers to a particular type of annotation associated with a base text. The name 'ruby' originated from a named font size (about half the size of the normal 10 point font) used by British typesetters.

Typically ruby is used in East Asian scripts to provide phonetic transcriptions of obscure characters, or characters that the reader is not expected to be familiar with. For example it is widely used in education materials and children’s texts. It is also occasionally used to convey information about the meaning of ideographic characters.

In Japanese, where ruby is called furigana, phonetic transcriptions typically appear in hiragana above horizontal text and to the right of vertical text. Although ruby in Japanese is often in hiragana, it is also possible to occasionally find annotations in kanji, katakana (particularly in Japanese 'manga' comics) and Latin text.

Furigana typically appears below horizontal text, and to the left of vertical text on the rare occasions that it is used to express semantic information.