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A gerah (Hebrew: גרה) is an ancient Hebrew unit of weight and currency, which, according to the Bible, Exodus, 30:13, was equivalent to 1/20 of a shekel. God tells Moses, the payment for life ransom during the census taking is 1/2 a shekel, "which weighs ten gerahs". This would make a whole shekel equal to 20 gerahs.
A gerah is in Aramaic a ma'ah "מעה" (Mishnah Hebrew pl. ma'ot "מעות" which means "coins"). It was originally a fifth of a Denarius/zuz, as seen in Exodus ("20 gerah is a shekel"), then became a sixth of a denar/Zuz, such as the Yehud coins which came in two denominations, approximately .58 gram as a ma'ah and approximately .29 gram as a half ma'ah (chatzi ma'ah), and (.58 X 6 = 3.48) which is about the weight of a Zuz/Denarius based on a 14 gram Shekel.
The Jerusalem Talmud Shekalim, in the Mishnah, debates if a kalbon, which was added when annually giving a half shekel to the Temple, was a "ma'ah" or a "chatzi ma'ah" (half ma'ah).
See also
References
Exodus 30:13 Book of Numbers 3:47