Particle の omitted when writing but not when saying


There are words, like 箱根関（はこねのせき）, 伊勢国（いせのくに） or 加賀国（かがのくに）that despite being
written without a particle の before 関 or 国 in the previous examples, this
particle の is said when reading them because it exists an invisible particle の
so that you must say it.

I'm sorry for explaning this in such an unpolished way, but because of that,
my question is: **how is called in Japanese grammar/linguistics this
phenomenon of "the invisible の that is omitted when writing but not when
saying"?**

**And do these words where this phenomenon occurs receive a name in
Japanese?**




This is an obsolete convention which you will only find with relatively old
place names (and occasionally surnames). Basically everyone is aware of this
phenomenon, but I don't think there is a well-known name for it. I think it's
because this is nothing more than an outdated convention rather than part of
the modern standard grammar. Place names are always the source of
unpredictable kanji readings, anyway. All we can do is to recognize this
pattern and remember the reading of each phrase individually.

Sometimes, exact the same place name can be written with or without の. A well-
known example would be 三ノ宮 vs 三宮:

> #### [Sannomiya Station (JR
> West)](https://en.wikipedia.org/wiki/Sannomiya_Station_\(JR_West\)#Etymology)
>
> All of the other transportation facilities in Sannomiya are written in
> Japanese as 三宮, without the Katakana character "ノ". Only JR includes it in
> the name, written as 三ノ宮駅. It is thought the reason it was included was to
> prevent people traveling from other parts of the country from misreading the
> name.

