Kilayim, Chapter Two, Mishnah Ten. In the previous Mishnah we studied the view of the Sages, that one who sows a field the size of a beis se'ah may plant within it nine garden beds of different kinds of vegetables or different kinds of grain. The nine beds do not fill the entire area; a gap remains between them. Although there was a dispute as to exactly how the beds are to be arranged, according to all opinions a gap of a beis rova is required - the area needed for sowing a quarter of a kav, which is about ten cubits by ten cubits - between the beds of grain that are sown.
Our Mishnah teaches something new: those areas of a beis rova that separate between the different kinds of grain need not themselves be fit for sowing. The mere existence of an empty separating space is enough.
"Kol shehu besoch beis rova oleh bemidas beis rova":
Anything found within that area of a beis rova, the roughly ten cubits by ten cubits that separate between the grains, "oleh bemidas beis rova" - counts toward the measure of the beis rova required between each of the nine beds. The Mishnah then lists examples of things that are not fit for sowing:
"Achilas hagefen" - the space taken up by a vine. When a vine grows in a field, one may not sow within an area of six handbreadths around it. Therefore, if a single vine stood within that empty plot, it is impossible to sow within six handbreadths of it, and nevertheless the area counts as a separating beis rova.
"Vehakever" - the grave. An area containing a grave is likewise not fit for sowing, and yet it counts as a separation.
"Vehasela" - the rock. An area of a beis rova made of stone, in which sowing is certainly impossible, also counts as the separation between the different kinds of grain.
From separating nine garden beds to separating between two kinds:
From here the Mishnah moves on from the case of planting nine beds in a beis se'ah to a case in which a person wishes to sow two different kinds side by side.
"Tevuah bitevuah" - one who sows one kind of grain alongside another kind of grain must leave the measure of a beis rova between them: the area needed for sowing a quarter of a kav, which according to the Rambam is a square of about ten and a fifth cubits by ten and a fifth cubits. Once this gap exists at one point along the border between the two kinds, it is permitted to bring them closer together along the rest of the border between them; it is enough that there be one point that has an area of a beis rova.
The Mishnah mentions a further limitation: one kind may not be completely surrounded by the other kind on all four sides, but must be open on at least one side. When it is "chavush," that is, completely surrounded, it is not permitted even with a separation of a beis rova.
The measures of separation between the kinds:
"Yarak beyarak" - one kind of vegetable alongside another kind of vegetable. Unlike grain, which is a seed crop, here a separation of six handbreadths is enough.
"Tevuah beyarak veyarak bitevuah" - when grain and a vegetable are side by side, we follow the larger measure: six handbreadths are not sufficient, but rather a square area of a beis rova is required.
"Rabbi Eliezer omer: yarak bitevuah shishah tefachim" - Rabbi Eliezer disagrees and holds that a vegetable and grain together are also measured by the measure of a vegetable, just as a vegetable alongside a vegetable requires only a separation of six handbreadths. In his view, even for a vegetable alongside grain there is no need for a beis rova, and six handbreadths between them is enough.
In summary: In this Mishnah we learned that the area of a beis rova that separates between the kinds need not be fit for sowing, and whatever is found within it - the space of the vine, the grave and the rock - counts toward the measure of a beis rova. We also learned the measures of separation between two kinds sown side by side: grain alongside grain requires a beis rova (at one point, provided that one kind is not surrounded by the other on all four sides), a vegetable alongside a vegetable requires six handbreadths, and grain alongside a vegetable requires a beis rova - and according to Rabbi Eliezer, only six handbreadths.