31 Halachos L’Moshe MiSinai according to the Rambam

“Halacha l’Moshe MiSinai” is a term used to refer to a law communicated by God to Moses that has no reference in the written Torah. These laws are universally acknowledged and unanimously agreed upon; the Sages of the Mishna and Talmud have no disagreements about halachos l’Moshe miSinai.

First an example of what a halacha l’Moshe miSinai is not: the Torah tells us to abstain from labor on Shabbos, but it does not define for us what labor is. The details of the 39 melachos (categories of Sabbath labor) are given in the oral law but this is not a halacha l’Moshe miSinai because it has a reference in the Torah. Similarly, how to perform shechitah (ritual slaughter), that “an eye for an eye” means financial compensation, and that an esrog is what the Torah means by “the fruit of a goodly tree” are all important laws clarified orally by God to Moshe but they are not halachos l’Moshe miSinai.

In the introduction to his commentary on the Mishna, the Rambam lists 31 halachos l’Moshe miSinai, which he says are most of them, if not all of them. These are:

1. That the loaves of a thanksgiving offering need a half-log of oil; 2. That the offering upon completion of a nazir period requires a quarter-log of oil; 3. That there is an eleven-day period of ritual purity between menstrual periods; 4. The concept of “gud asik,” legally considering walls to be extended; 5. The concept of “lavud,” legally considering small gaps to be closed; 6. The concept of “dofen akuma,” that a ceiling is considered the bent extension of a wall; 7. Minimum sizes, such as of food for blessings; 8. The minimum size of a chatzitzah, an interposition between a person and a mikvah; 9. The minimum dimensions of a partition; 10. The parchment to be used for tefillin; 11. The parchment to be used for mezuzos; 12. The parchment to be used for Torah scrolls; 13. The shape of the letters Shin on tefillin; 14. The shape of the knots of tefillin; 15. That the straps of tefillin must be black; 16. That tefillin are cube-shaped; 17. That tefillin have slits for the straps; 18. That tefillin parchments must be rolled and tied with hair from a kosher animals; 19. That tefillin compartments must be sewn with thread from kosher animals; 20. The ink to use for writing a sefer Torah; 21. The quill to use for writing a sefer Torah; 22. That a girl violated until the age of three is still legally considered a virgin; 23. How to determine how much peah must be given from a field containing several types of grain; 24. That the amount of kilayim (diverse species) that may not be plowed if planted accidentally is 1/24 that which would invalidate an entire field; 25. That a field with ten or more saplings may be plowed right up until Shemittah; 26. That if a food item is partially impure, one may take terumah from the pure portion for both the pure and impure parts; 27. That the laws of orlah (the fruit of a tree’s first three years) apply in all places; 28. That a teacher may use an oil lamp to find his students’ place in a text on Shabbos; 29. That a woman is liable even for carrying in the back of her apron on Shabbos; 30. That a wine merchant is permitted to mix strong and weak wines together; 31. That Jews who live in Ammon and Moab must give maaser ani (tithe for the poor) every seven years.

Halachos l’Moshe miSinai are typically introduced by the words “b’emes omru” (“in truth, they said”), though the phrase does not necessarily always indicate a halacha l’Moshe miSinai. Furthermore, the phrase “halacha l’Moshe miSinai” is sometimes used to identify laws that are ancient and universally accepted like actual halachos l’Moshe miSinai. Even in the Rambam’s list, at least two of the “halachos l’Moshe miSinai” (#28 and #30) appear to refer to rabbinic enactments.