I have been searching for an official version of the Pinyin chart and I came across multiple variations of it. One of the charts I saw had the combination "yo". A quick search showed that one character it is used with is 唷 (yō).
The definition of 唷 is:
(interjection expressing surprise) Oh!
My question is, why is "yo" omitted from some Pinyin charts? Is it a rare combination or is there any other reason? (Judging from the above definition, it seems like the hanzi is used frequently enough?)
That is but one inconsistency. There are many more differences from one chart to the other (For example, some charts don't have "yun" but there are clearly hanzi that use it). Is there an official set of combinations? (I found some compilations of the combinations but I don't know if some initial-final combinations are in use now.)
P.S. I read an article about "er" being part of a group of four special vowels and about how erroneous it was to place it in the chart with other pinyin finals. Does anyone know more about this? I can't find the link for the article so I don't have enough information to create a new question.