You’ll never see these four hiragana on license plates, but maybe not for the reasons you’d expect….
License plates may not be the most exciting things on Earth, but the rules and regulations that go into them do have some interesting exceptions. For example the vanity plate “ILVTOFU” is banned in many states in the U.S. even though some people just want to express their love of tofu, as is “370H55V” for spelling out a not-so-nice word when read backwards and upside down.
And Japan has its own license plate exceptions too. Vanity plates are not really a thing in Japan, leading to most license plates having the same basic structure: the issuing office, a hiragana, and then some numbers.
▼ A license plate from Sendai with the hiragana “a” followed by the numbers 20-19.
MagX（ニューモデルマガジンX） (@CyberMagazineX) December 13, 2016
The usage of hiragana on license plates is unique to Japan, though out of all possible hiragana that could be used, there are some that never appear on license plates. They are:
1) Any hiragana with ten-ten voicing marks on the side (such as が (ga), じ (ji), ど (do) etc.)
2) Combination hiragana sounds (such as きゃ (kya), ちょ (cho), みゅ (myu) etc.)
3) Old hiragana not used anymore (such a ゐ (wi) and ゑ (we) etc.)
4) わ (wa) and れ (re) are only used on rental cars.
That still leaves a bunch of hiragana left over that can be used, but among them there are four which – for various reasons – are not allowed to be used on license plates. They are:
1) お (o). This one isn’t used because of its similarity to the hiragana あ (a). During a high-speed chase, the last thing police want to do is have to squint and try to deduce if they’re tailing an お or an あ only to get it wrong. This might also be why the similar-looking わ and れ are both only used for rental cars too.
2) し (shi). し isn’t used because it’s pronounced the same as the word 死 (“death”), making it an unlucky hiragana to have. Those who have studied Japanese know that the number four (also pronounced the same) is considered an unlucky number, similar to 13 in the West. But still, there are plenty of license plates with 13 in Western countries, and their owners haven’t all faced untimely deaths… probably.
3) ん (n). This one kind of makes sense, seeing as ん is the only hiragana that is just a consonant and not a consonant followed by a vowel. There are no words in Japanese that start with ん making it difficult to pronounce on its own. When department store clerks announce over the loudspeaker that a car has its lights on in the parking lot, they want to be able to get that info out clearly, not stumble over pronunciation errors.
4) へ (he). And here we have the strangest exclusion of them all. The reason you’ll never see a へ on a license plate? Because it sounds the same as 屁 (“fart”). We assume that policemen and store clerks would just burst into a fit of non-stop giggling if they had to read a license plate with へ on it, which led to its banning. Although perhaps the even funnier image is a room full of policy-makers deciding which hiragana are okay to use on license plates actually discussing the fact that へ makes them think of farts and striking it down because of that.
Thinking about it, it’s unfortunate that some hiragana are banned because of their similarity to other words. Personally I know that I would love to have an Evangelion license plate with し (“death”) on it, or a cute little farmer and cow license plate with へ (“fart”) on it just for giggles.