A Ruby is part of an annotation pair - a piece of
text ("Ruby Text") that is associated with another piece of text known as
a "Ruby Base". A common need for such a structure arises in some asian
writing systems where a ruby association is used to present helper readings
for uncommonly used writing characters.
With a character set as complex as Chinese or Japanese, some characters are used
rarely and are thus not as easily recognizable by younger children or possibly
many adults. In Japanese writing, for example, the phonetic Hiragana alphabet
is used to pair phonetic 'helper' readings (called Furigana or Yomigana in
Japanese) with the Chinese character counterpart.
A Ruby text will usually be rendered next to the Base text such that the
association between each Base text and Ruby text is clear (often directly
above the Base text.) Ruby text is usually also rendered in a smaller font
than the Base text to help visually clarify which part is which in the
The Ruby Text Container (RTC element) is part of the Complex Ruby markup,
where as many as two Ruby text annotations (RTC elements) can be bound to
a single piece of Base text. The RTC element serves as a container for Ruby
Text (RT) elements. Complex Rubies also allow Base text and Ruby
text to be broken down further, so that even more detailed associations
can be made between them.
Complex rubies can have multiple Base elements, and multiple Ruby Text
components. In a Complex Ruby, all of the Base elements (RB)
are enclosed in an RBC element (RBC: Ruby Base
Container.) For the Ruby Text elements (RT), these
are enclosed in an RTC element (RTC: Ruby Text Container.) A Complex Ruby
can have one or two RTC elements which allows for two text associations
to be given for any given Base text. Default rendering might render the
first RTC group ABOVE the Base text, with the second RTC group
possibly rendered BELOW the Base text... but, this rendering is
definitely not guaranteed - allowances for alternate writing directions
(eg: vertical rather than horizontal) and other criteria can complicate
rendering, and CSS properties have already been proposed specifically for
Ruby markup in future CSS versions to control these details.
The content of the Ruby Base Container (RBC) is, of course, RB elements.
Similarly, the content of any Ruby Text Containers (RTC) are RT elements.
With Complex Rubies, RT elements have an additional attribute RBSPAN which
acts like COLSPAN does for TD and TH elements in tables - it allows a
piece of Ruby Text content to be associated with multiple RB base elements.