Negaim, Chapter 12, Mishnah 4. This perek treats nigei batim, the tzara'as that breaks out on a house. We have already learned that only a house whose walls are constructed of three substances can contract this tumah: avanim, afar and eitz, stones, earth and wood, and every one of the walls must contain all three. Now the Mishnah turns to the first of those substances and asks what quantity of stone a house must have before tumah can take hold in it.
The Rambam's preface: a shiur of two grisim
Before entering the Mishnah itself, it pays to hear how the Rambam frames the entire discussion in his Peirush Hamishnayos. There is a fact, he writes, that one has to know at the outset: in nigei batim the measure of the nega is two grisim. Why two? Because the pasuk that describes the kohein's inspection states the word nega twice within one verse ("vehinei hanega"), and from that repetition the Sifra derives that a house is never declared tamei for anything less than a doubled measure, two grisim.
That settles the quantity. What our Mishnah comes to debate is how the quantity has to be laid out: across how many stones, and in which part of the wall.
Rebbi Yishmael: four stones
The Mishnah asks, "Vechamah avanim yehu vo," and how many stones does the house need to have? "Rebbi Yishmael omer, arba," Rebbi Yishmael sets the number at four, and his reasoning is recorded: nothing happens "ad sheyeira'eh kishnei grisim," until an area the size of two grisim becomes visible, whether spread "al shtei avanim," over a pair of stones, "o al even achas," or gathered onto a single stone.
Rebbi Yishmael demands two grisim in each wall, and he demands that each wall have at least one stone bearing the nega. Since, however, he lets both grisim rest on that one stone, the house can reach the shiur with a lone afflicted stone in every wall: four walls, one stone apiece, two grisim on each of those stones, giving a minimum of four stones and eight grisim in the house.
Rebbi Akiva: eight stones
"Rebbi Akiva omer, shemonah," Rebbi Akiva puts the figure at eight, and here too his reason is stated: the two grisim have to be seen "al shtei avanim," divided between two stones, and "lo al even achas," not concentrated on one. To Rebbi Akiva, the two grisim of a wall may never be crowded onto the same stone. Every wall consequently requires a pair of stones, each carrying a gris of the nega, and four walls with two stones each add up to eight stones in all.
Rebbi Elazar ben Rebbi Shimon: eight stones at the corners
A third view is then recorded, that of Rebbi Elazar ben Rebbi Shimon. He too speaks of two grisim appearing on two stones, but he adds a condition: "kishtei ketzavos bazaviyos," in the form of two strips at the corners, "orko kishnei grisim verochbo kegris," the nega measuring two grisim along its length and one gris across its width.
The Rambam points out the source of this reading. The Torah places the nega "bekiros habayis," among the house's walls, and Rebbi Elazar ben Rebbi Shimon takes that to indicate the spots where one wall joins another, that is, the corners. Picture a band of nega a gris wide that starts on the corner stone of one wall and carries on around the angle onto the corner stone of the wall meeting it at a right angle: the band runs two grisim in length and one gris in breadth, straddling the corner. A house has four corners, so this opinion likewise arrives at eight stones. Worth noting is that this is the shitah Rashi adopts in his peirush on Chumash.
The halachah
We rule like Rebbi Akiva: eight stones, a pair in each of the four walls, each stone of the pair bearing a nega the size of a gris, with no requirement that they be positioned at the corners.