In linguistics, Dutch is in the same subgroup, namely West Germanic Languages, of the Germanic Language family. Apart from Dutch, this subgroup includes two of the most spoken languages in the world: English and German. Similar to English, the Dutch alphabet has 26 letters, and the last 3 are x, y, z.
However, in Dutch, the letter y is sometimes written as ij. They are homonyms (both pronounced like /ei/), and should be interchangeable in almost all situations. Just like the letter ß in German, ij is not only a ligature, but also a/one letter. Many believes that ij should be the 27th letter of the Dutch Alphabet, or ij should completely replace y.
It is not clear now whether the claims would be ever considered, but the omitting of y is overwhelmingly accepted in primary education. As a vowel, ij looks more comfortable than y. For example, ijsbeer (polar bear) is easy to understand for six-year-old. But ysbeer, or any word starting with y seems a bit weird, and it becomes only weirder when surrounded by consonants.
Another situation where ij is widely adopted is the names of people and places, especially in those with traditions:
IJlst(city in Friesland)IJssel(river in the Netherlands)
An interesting phenomenon in the above examples is that both i and j are capitalized. It further supports the argument that ij is one letter. The phenomenon is so special because it is unparalleled among all mainstream European languages. The German ß is considered as a letter, but it is never used at the start of a word. The Spanish ligature ll is no longer considered as a letter. At the start of a word, only the first l should be capitalized, like Llamar (to call).
On the other hand, ij in names are sometimes deliberately changed to y as a part of the anglicisation process. The change is mainly for commercial and globalization purposes. Examples are:
Ruud van Nistelrooy(ex football player, formerlyNistelrooij)Feyenoord(football club, formerlyFeijenoord)Spyker(car manufacturer, formerlySpijker)