There is a similar thing with Hebrew numerals¹ where 15 and 16 are usually written as תו and תז to avoid what would otherwise be forms of the name of God.
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1. The Hebrew numeral system, like the Greek numeral system is an almost-decimal system in that different letters are used for each of the different values, but the letters change by place value as well as by their individual value, so, e.g., 21 is written כא where כ stands for 20 and א stands for 1.
Interesting. I wonder if this is related to how Germans count/pronounce numbers. 21 in your example is "einundzwanzig" in German (or "ein-und-zwanzig", "one-and-twenty")
D’oh that’s what I get for trusting my mediocre Hebrew keyboarding skills. For those who don’t know the Hebrew alphabet, both ת and ט are commonly transcribed as T (the former often as TH, but it’s meant to be a German aspirated T sound rather than the English th).
The only Japanese number form change I’m aware of is a special form for one (一) (and possibly two 二) to prevent them from being altered on checks into three (三), but I’m far from an expert.
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1. The Hebrew numeral system, like the Greek numeral system is an almost-decimal system in that different letters are used for each of the different values, but the letters change by place value as well as by their individual value, so, e.g., 21 is written כא where כ stands for 20 and א stands for 1.