> IJ probably developed out of ii, representing a long [iː] sound [...]. In the Middle Ages, the i was written without a dot in handwriting, and the combination ıı was often confused with u. Therefore, the second i was elongated: ıȷ. Later, the dots were added, albeit not in Afrikaans, a language that has its roots in Dutch. In this language y is used instead.

> Alternatively, the letter J may have developed as a swash form of i. In other European languages it was first used for the final i in Roman numerals when there was more than one i in a row, such as iij for "three", to prevent the fraudulent addition of an extra i to change the number. In Dutch, which had a native ii, the "final i in a row elongated" rule was applied as well, leading to ij.

> Another theory is that IJ might have arisen from the lowercase y being split into two strokes in handwriting. At some time in the 15th or 16th century, this combination began to be spelled as a ligature ij. An argument against this theory is that even in handwriting which does not join letters, ij is often written as a single sign.

So indeed maybe the same as the Roman numerals. Interesting, I didn't know this. (Also interesting that no one really knows where this letter came from.)

It doesn’t undergo the phonetic process of umlaut, Don, and as it’s a vowel the two dots are more like a simple diaeresis.

It is used in that way in French too.

It is neither umlaut nor diaeresis, but just an i and a j with each their own dot smushed together into a ligature, which can take either the form of a y with both those dots on top, or a more rounded form that more resembles an i and j joined together.

I guess in other languages than Dutch it could be a y with a diaeresis or umlaut.

Verbal diaeresis?