Bava Basra, Chapter 7, Mishnah 1. This Mishnah also deals with the laws of sales and the meaning of certain expressions in the language of a transaction. The Mishnah opens: "Ha'omer lachaveiro beis kor afar ani mocher lach" - A person who says to his friend that he is selling him a plot of land the size of a beis kor.
What is a "beis kor":
Se'ah - A unit of volume. It can be compared to a six-pack of two-liter bottles, meaning about twelve liters.
Kor - Thirty se'ah, about three hundred and sixty liters. This is the largest measurement mentioned in the Mishnah.
Beis kor - The area of land in which one can plant a kor of barley seeds. Its size is about seventy-five thousand square amos, roughly five acres (about twenty dunams), about the size of three soccer fields. This is a very large plot of land.
It is worth paying attention to the seller's wording: He did not say "a beis kor of ground" but rather "beis kor afar", which means land suitable for planting, where crops can be grown.
Clefts and rocks:
The Mishnah continues: "Hayu sham nekaim amukim asarah tefachim o sela'im gevohim asarah tefachim, ein nimdadin imah" - if there were deep clefts ten tefachim deep or rocks ten tefachim high, they are not measured with it. This means, if within that same land there are depressions and crevices ten tefachim deep (and the Gemara explains that this also involves a width of four by four tefachim), or rock protrusions that are ten tefachim high and four by four tefachim long and wide.
The reason for this: In these obstacles, the pits and rocks, it is not possible to plant. Since the seller committed to selling "earth" suitable for planting the size of a beis kor, those parts are not counted in the overall area. Therefore, he must deliver land the size of a beis kor, plus any addition required to compensate for the parts that are not suitable for planting due to the clefts and rocks.
On the other hand, had he said "a beis kor of ground", it is obvious that in such a huge area there will be rocks or depressions where it is impossible to plant, and they are included in the transaction. The Gemara establishes that as long as the area unsuitable for planting is less than one in forty-five (about 2.22 percent of the area), everything is included in the sale.
"Pachos mikan nimdadin imah":
If the obstacles - the depressions in the ground or the rocks above it - do not reach the measure of ten tefachim, in the height of the rocks or the depth of the clefts, they are included in the measurement of the beis kor.
"Ve'im amar lo k'veis kor afar":
When the seller did not say "beis kor afar" but rather added the letter kaf of comparison, a wording that indicates an approximate measure and not an exact quantity, the law changes: "Afilu hayu sham nekaim amukim yoser measarah tefachim o sela'im gevohim yoser measarah tefachim, harei elu nimdadin imah" - even if there were clefts deeper than ten tefachim or rocks higher than ten tefachim, they are measured with it. The pits and protrusions are included in the sale and are considered part of the land, since from the wording "like a beis kor" it is understood that the area will include a crack or rock here and there, and as long as this is within the margin of error of less than 2.22 percent, it does not constitute a flaw in the transaction.
In summary: In this Mishnah we learned the exact meaning of measurement expressions used in land sales. "Beis kor afar" means land suitable for sowing in an exact measure, and therefore crevices ten tefachim deep and rocks ten tefachim high are not measured with it, and the seller must make up the difference. If they are less than this measure - they are measured with it. However, with the expression "k'beis kor afar", which is an expression of an approximate measure, even crevices and rocks ten tefachim deep or high are measured with it and included in the sale.