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Who knows eighteen?

Please cite/link your sources, if possible. After about one business day, I will:

  • Upvote all interesting answers.

  • Accept the best answer.

  • Go on to the next number.

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14 Answers 14

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Eighteen are the minimum (or according to Rambam, the absolute) number of days in a woman's cycle, according to the original Torah law: seven of niddah (Lev. 15:19), and eleven of zivah (based either on Lev. 15:25, or an unwritten halachah le-Moshe mi-Sinai - Talmud, Niddah 72b-73a).

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Shemona Asar was the number of berachot in the Shemona Esrei, before the addition of Birkas HaMinim." (Berachot 28b)

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Eighteen is the value (in maneh) of the linen clothing that the Kohen Gadol wore for his first entry into the Holy of Holies each Yom Kippur, according to the majority opinion (Yoma 34b).

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Eighteen are the years before one gets hitched. (Ben Shemonah Esrei LiChupah: Avos 5:21)

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  • 2
    Does? Should? May?
    – Isaac Moses ♦
    Apr 16, 2010 at 17:09
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    I heard the Biblical hint for this is that "Adam" appears 18 times before the first use of "Ish"
    – Shalom
    Apr 16, 2010 at 18:39
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    Another one is that the verse about who the Kohen Gadol may marry (Lev. 21:13) starts with the word והוא, whose numerical value is 18 (Baal Haturim on this verse).
    – Alex
    Apr 16, 2010 at 19:54
  • Isaac. it is too complicated to get into and besides, none of those words fit the meter of the tune!
    – Yahu
    Apr 19, 2010 at 3:53
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    Checkmark due to value added by interesting comments.
    – Isaac Moses ♦
    Apr 19, 2010 at 17:46
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There are eighteen sins for which the punishment is misa bide shamayim but for which one can get lashes. Source: Rambam, Sanhedrin 19.

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18 are the instances of prayer in the Torah, as enumerated by Rav Saadya Gaon in the introduction to his Siddur: Gen 20:17, 25:21, 32:10, Exo 8:8, 8:26, 9:33, 10:18, 15:1, 32:11, 33:13, 34:9, Num 11:2, 12:13, 14:13, 21:7, 21:17, 27:16, Deu 3:23.

He also mentions that there are 18 days the Torah sanctified as Moadim: 7 for Pesach, 1 each for Shavuot, Rosh haShana, and Yom Kippurim, and 8 for Sukkot.

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18 things were enacted at this day (Mishna Shabbat 1, 4)

וְאֵלּוּ מִן הַהֲלָכוֹת שֶׁאָמְרוּ בַעֲלִיַּת חֲנַנְיָה בֶן חִזְקִיָּה בֶן גֻּרְיוֹן כְּשֶׁעָלוּ לְבַקְּרוֹ. נִמְנוּ וְרַבּוּ בֵּית שַׁמַּאי עַל בֵּית הִלֵּל, וּשְׁמֹנָה עָשָׂר דְּבָרִים גָּזְרוּ בוֹ בַיּוֹם: ‏

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There are 18 perakim in maseches Oholos.

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Eighteen are the rows of R. Elazar Ben Azaria's beard that turned white.

(Berachot 28a)

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Lag B'Omer is on 18 Iyur

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    Only in our fixed calendar (or in the variable calendar if Nisan happened to have 30 days).
    – Double AA ♦
    Feb 14, 2013 at 23:09
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Eighteen is the age at which one should finish learning the entire Talmud according to one opinion (15 to start learning (Pirkei Avos), and it is a machlokes (in kiddushin I think) if it takes 3 or 5 years to complete it).

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    Sourcing that machloket in kiddushin would be valuable. (Even better would be to show that the opinions in kiddushin had ever seen the pseudo-mishna in avot about ages.)
    – Double AA ♦
    Feb 2, 2014 at 5:57
  • @DoubleAA pseudo-mishna?
    – Asher
    May 25, 2020 at 2:58
  • @Asher it's one of those braysas that often gets printed after Avot (there's some after bikkurim too)
    – Double AA ♦
    May 25, 2020 at 3:00
  • @DoubleAA That's the 6th perek while this is the end of the 5th perek. i guess it could depend if the yehi ratzon is before or after this mishna-i think the gra puts it after the perek
    – Asher
    May 25, 2020 at 3:13
  • @Asher the fact is in all the old manuscripts of mishna, Avot ends before this.
    – Double AA ♦
    May 25, 2020 at 7:03
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18 [tefachim] was the height of the Menora.

(Menachot 28b)

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Sichon's (the King of the Emorites) thigh bones were eighteen cubits long (Midrash Tanchuma, Buber ed., Devarim appendix 7)

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18 is the gematriya of chai

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    Also of "Yach".
    – Double AA ♦
    Mar 25, 2012 at 6:09
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    Also of חטא.​​​​​​​​​​​​​​​
    – Double AA ♦
    Feb 15, 2013 at 4:28

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