Before the adoption of #ArabicNumerals into Hebrew script, the standard way to write numbers in Hebrew was with #HebrewNumerals, which work much like #LatinNumerals (y'know, V = 5, X = 10, so XV = 15). Rather than separate number symbols, numerical values were assigned to letters of the alphabet. Writing numbers meant writing a particular string of letters.

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Tu B'Av = "fifteen in Av" (i.e. `5th of Av)

But the Hebrew word for 15 is chamisha-'asar (literally "five-ten") so what's "tu"?

Before the adoption of #ArabicNumerals into Hebrew script, the standard way to write numbers in Hebrew was with #HebrewNumerals, which work much like #LatinNumerals (y'know, V = 5, X = 10, so XV = 15). Rather than separate numerical symbols, numerical values were assigned to letters of the alphabet. Writing numbers meant writing a particular string of letters.
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