Chapter 1, Mishna 1
From where do the Sages derive their units of measurement? Some from Written Torah, some from Oral Torah. The seven species which characterize the abundance of the Land of Israel – wheat, barley, grape, figs, pomegranates, olives and dates – are all the basis of measurements as well. The command to use them as measures was transmitted l’Moshe miSinai, to Moses at Sinai, as part of the Oral Torah, along with the laws of interpositions that invalidate immersion in a mikveh, ritual bath, as well as the law of mechitzos, partitions. Do dreadlocks invalidate a mikveh immersion? How do we know that a valid partition between domains must be ten handbreadths high? From the Ark of the Covenant which contained the Tablets of the Ten Commandments, and into which the Divine Presence would descend. We know the crossbeam over a mavoi, alley, must be 20 cubits above the ground or less to allow carrying there on Shabbos. What if it can’t be lowered? Then we raise the ground!