realfish
commented
3 years ago For vertical Chinese text, the Bopomofo annotation is set as same as other phonetic annotations like Hanyu Pinyin for Chinese (as well as Furikana for Japanese), which is by default set on the block-start side logically, i.e. the right side visually.
In metal typesetting, the Bopomofo annot sometimes was preferred to be founded into a single sort with the annotated base character. At first, this kind of sorts was supposed to be used in vertical layout, thus the Bopomofo was located on the right side. Nevertheless, they were also used to set horizontal text for later practice. Such a technical twist was inherited even by the digital type foundries today. For example, there is a pack of fonts AR Zuinn in stock from the Arphic Types (文鼎字型), which includes the options that combine the Bopomofo annot and the base char into a single sort (see and search 國字加注音字型). This pattern of typesetting could be officially used in the formal textbooks especially for primary education in Taiwan (see 國語注音符號手冊 which is also referred by CLReq).
It makes the right-side setting Bopomofo annotation a de facto standard, for both vertical and horizontal layouts. The inter-char ruby is supposed to meet the above Bopomofo annot requirement for horizontal layout.
It should be considered that the right-side Bopomofo is derived from the movable type traditions, thus the right side setting is both visual and physical. In the other hand, the single-line Chinese text layout direction is quite free and arbitrary, i.e., top-to-bottom (normal vertical), left-to-right (normal horizontal), right-to-left (horizontal setting for transportation scenario, and may also be treated as a special case of vertical-rl layout whose column width is one em / one ideographic width), and bottom-to-top (vertical setting for transportation scenario). For all the cases mentioned above, it should be fine enough to use the right-side Bopomofo annot as the default.
Inter-character bopomofo annotations are go on the right side of the character they annotate. Is it the same in right-to-left Chinese? (I think so, but I would like to confirm it with the TF.)