When I stopped at Yotsuya (四ツ谷) Station this summer, I noticed a katakana tsu (ツ) in the station name.
Is there a reason why the katakana character is used (instead of a hiragana one)?
Before the modern kana usage (現代仮名遣い) has permeated through the nation, it was very common for katakana ツ to be used after a numeral kanji.
Japanese traditional numerals are:
1 - ひとつ
2 - ふたつ
3 - みつ ／ みっつ
4 - よつ ／ よっつ
5 - いつつ
6 - むつ ／ むっつ
7 - ななつ
8 - やつ ／ やっつ
9 - ここのつ
10 - とお
And today, these are written 「一つ、二つ、三つ、四つ、五つ、六つ、七つ、八つ、九つ、十」 (generally in vertical writings) or 「１つ、２つ、３つ、４つ、５つ、６つ、７つ、８つ、９つ、１０」 (generally in horizontal writings).
However, Japanese people who lived in the Edo period (1603 - 1868) wrote these traditional numerals usually using katakana ツ like so:
There are numerous books published in the Edo period which can prove that the number-ツ notation was common. Here is an example.
This image shows the last page of a textbook for children published in 1850. In the left page, there are two poems for fun. The first poem says:
And the second poem says:
The numerals express time in these poems. The tsu after the number kanji is always written by using ツ, not つ. This kind of ツ was rarely omitted in kana-kanji writings, so if a number kanji is not followed by ツ, the kanji is read differently. For example, the 五七 in the second poem would be read ごしち, not いつつななつ.
So, the number-ツ notation was Edo-period people’s general knowledge. Then, let’s see 四ツ谷.
It would presumably be natural for Edo-period people to place ツ between 四 and 谷.
In maps created in the Edo period, both 四ツ谷 and 四谷 are commonly used. However, relatively newer maps (created in 18th or later) and books introducing the famous sights tend to use 四ツ谷 more than 四谷.
For example, a map called 『江戸切絵図・四ツ谷絵図』 (published in 1849-1862) uses 四ッ谷 for the region name, and uses 四谷 for addresses.
Note that there are other maps using 四ツ谷 for addresses like 四ツ谷二丁目, 四ツ谷傳馬丁一丁目 etc., so it does not mean that whether to use 四ツ谷 or 四谷 depends on whether it’s a region name or an address name, at least in Edo-period cases. Today, 四ツ谷 is used for the station name and the so-called region name, and 四谷 is used for the official address name, though.
The region name written in the red square box in the ukiyoe picture is 「四ッ谷内藤新宿」. It’s using ッ.
And this is a Hiroshige II’s book introducing the sights of Edo including 四ツ谷 region.
The title of the 四ツ谷 part in it is 「四ッ谷御門外」. It’s using ッ as well.
So, these kinds of Edo-period printed products seemed to spread 四ツ谷 more than 四谷, and the majority of Edo-period people seemed to accept the ツ usage and generally used 四ツ谷 or 四ッ谷.
I don’t know who decided to use 四ツ谷 for the station name, but I can imagine why the decider chose 四ツ谷 rather than 四谷. Because most of the Japanese people were familiar with 四ツ谷 and actually used 四ツ谷, more than 四谷, at that time. The original 四ツ谷 station was opened in 1894. It was before the modern kana usage (現代仮名遣い) has permeated, so.
During the Edo period, katakana was commonly added to some kind of kanji to specify how to read the kanji. Many kanji letters have several ways to be read. Edo-period people seemed to try to avoid misreading by using katakana helpers in kana-kanji writings.
For example, in a case, they wrote 下 in this way:
下 can be read した, しも, か, げ, etc. But in this case, the author wants the readers to read it しも.
In some Edo-period books, a region name 新橋 is written like this:
So this 新 is not read あらた, にい, or さら. The ン indicates that the 新 is read しん.
ツ following a numeral kanji might be this kind of helper katakana as well.
Anyway, this kind of helper katakana custom no longer exists today. They are just wrong in the modern kana usage world. But some expressions of them has been surviving in proper nouns, like 四ツ谷.
If you travel around Japan, you would see a lot of them (the old katakana usage in proper nouns) surviving, especially number-ツ-pattern ones. For example, 一ツ橋、二ツ池、三ツ峠、四ツ岳、五ツ橋、六ツ美、七ツ釜、八ツ橋、九ツ山、etc. Most of their traditional names were born before the modern kana usage spread. So they are genuine survivors. But, some of those names were created by today’s people to express retro nuance or something. 四ツ谷 is genuine.
By the way, this answer focuses on ツ following a numeral kanji. I think that ケ of 関ヶ原, ノ of 三ノ宮 or other expressions like these have different origins. So, please don’t apply this answer to all kinds of katakana-between-kanjis cases.
I found a very long answer on Yahoo Chiebukuro:「東京の四ツ谷に勤務することになりました。疑問・・・」
which offers one theory that Yotsuya is originally an Ainu place name meaning "many valleys".
Whether Ainu or not, the "yomi" (pronunciation) came first, and then in the Meiji era the formal "hyouki" Kanji for public facilities were decided. As it happens, there are some minor variations in writing as described in this goo entry:
So actually it appears there is NOT actually always a katakana ツ。
In fact, the subway 四谷三丁目 also does not have a katakana ツ, even the journalist handbook says it does.
It is a difficult question for Japanese. Probably JR named their stations. In the Edo period, there were no ツ or ノ. But if there were no ツ or ノ, they would be read like 御茶水→おちゃみず, 四谷→よんたに or したに. So I think this is the reason why JR named them so.
It seems that while JR's station names did not change, place names changed 御茶ノ水→お茶の水, 四ツ谷→四つ谷→四谷, 青砥→青戸. There is a tendency for them to change to easier kanji as time passes.
Almost in old japanese, カタカナ are used. So the word 四谷 is old word, カタカナ is used.