Keilim, Chapter 17, Mishnah 8. Chazal fixed many halachot according to a shiur, a measurement taken from something familiar in the world: the volume of an olive, the width of a grain of barley, the size of a lentil, the thickness of a stick. But every one of those things comes in different sizes. Our Mishnah therefore steps back and tells us, for each of these standards, exactly which specimen the Sages had in mind.
The Mishnah begins with the olive. "Kazayit she'amru": whenever the Sages speak of the volume of an olive as a shiur, which they do in a great many halachot, they did not intend "gadol", an olive of the large sort, nor did they intend "katan", one of the small sort. Their intention was "benoni", an olive of intermediate size, and the Mishnah identifies it: "zeh aguri". Olives grow in many varieties around the world, and their fruit differs considerably from species to species. The aguri sits between the extremes, and Chachamim are teaching that one who gauges a kazayit by an aguri olive has used precisely the olive upon which the shiurim of Chazal were built.
Next comes the grain of barley. "Kese'orah she'amru": the barley kernel that serves as a shiur, as in the halachah that a bone from a human corpse as small as such a kernel conveys tumah, is again not to be taken at either extreme. Not "gedolah", the large kernel, and not "ketanah", the small kernel, but "benonit", the intermediate one, and the Mishnah names it: "zo midbarit". The reference is to the barley of the desert, the variety that grows in the wilderness.
The third measure is the lentil. "Ka'adashah she'amru": the lentil appears as a shiur in various halachot, among them the rule that a fragment of a dead sheretz of that volume conveys tumah. Here too the Mishnah rules out both ends of the range. Not "gedolah" and not "ketanah", but "benonit", the lentil of middling size, which the Mishnah specifies as "zo mitzrit", the Egyptian variety.
The Mishnah then turns to a different sort of shiur. "Kol hamitaltelin": every object capable of being carried, teach Chachamim, "mevi'in et hatumah", is able to bring tumah onto the one carrying it, but only "be'ovi hamardea", when the tamei object possesses the thickness of a mardea. The mardea was the rod a farmer gripped in his hand to prod his ox and keep it advancing, and such rods were made to a familiar thickness. A tamei item as thick as that rod could render the person transporting it tamei as well.
And once again the Mishnah declines to leave a measurement to guesswork. Not "gadol", a thick goad, and not "katan", a thin one, but "benoni", one of average thickness. Which goad qualifies as average? "Kol shehekeifo tefach": one that measures a tefach the whole way around. These rods were round, so the test is the distance around the pole; if wrapping it comes to a full tefach, we have a goad of the intermediate sort. And since we calculate that the width across a circle equals one third of the distance around it, such a rod is a third of a tefach across, and an object of that thickness is able to transmit tumah to whoever carries it.
In one short Mishnah, then, four of the Torah's working measures are given a precise identity: the aguri olive, the desert barley grain, the Egyptian lentil, and the ox goad a tefach around. Chazal did not leave their shiurim to the eye of the beholder. They pointed to a specific olive, a specific grain, a specific lentil and a specific stick, so that every Jew measuring a halachah would be measuring by the very same standard.