Kilayim, chapter 4, mishnah 3. Our mishnah cites the view of Rabbi Yehudah, who disagrees with the definition we learned in the previous mishnah regarding the question of what constitutes mechol hakerem. Rabbi Yehudah says: "ein zeh ela geder hakerem" - the case described in the previous mishnah, where according to Beis Hillel twelve amos are needed in the mechol hakerem in order to plant there, is nothing more than the fence of the vineyard: the space between the vineyard and the fence, as we described. This is not the mechol hakerem, which requires twelve amos.
The law of the vineyard fence according to Rabbi Yehudah:
By a vineyard fence, as long as a person sets aside four amos next to the vineyard, he may sow the rest, and it makes no difference how large the space is; he need not be concerned about it at all.
So what is the mechol hakerem?
Where, then, does the law of mechol hakerem apply, in which Beis Hillel ruled that twelve amos are needed in order to sow? Between two vineyards. That is, when two vineyards stand next to one another - and this is not an area that was destroyed, but two separate vineyards from the outset - at least twelve amos are required there: four amos for each of the vineyards as its 'working space', the area needed to work the vineyard, and another four amos in the middle, so that the place will be considered a field in its own right and not subordinate to either of the two vineyards.
Beis Shammai, of course, will say that sixteen amos are required there: two sets of four amos so that the place will be considered a field. That is the approach of Beis Shammai. But according to Rabbi Yehudah, the very definition of mechol hakerem - the case in which Beis Shammai and Beis Hillel disagreed - applies specifically between two vineyards.
The first tanna, who disagrees with Rabbi Yehudah, holds that even between two vineyards the area is considered a mechol hakerem, and according to the law of mechol hakerem Beis Hillel say that sixteen amos are needed and not twelve. It turns out that the law is not the same in both cases, and the measure of twelve amos is relevant only when we are dealing with the area next to the fence.
The words of the Yerushalmi:
The Yerushalmi mentions that if the vines of the two vineyards line up directly opposite one another, even Rabbi Yehudah agrees that the area between them has the law of mechol hakerem, since it emerges that these two vineyards are really nothing other than a single vineyard, and the area between them will have the law of a clearing within a single vineyard. Only when the vines do not line up opposite one another, where it is clear that these are two separate vineyards, does the law of twelve amos apply according to Rabbi Yehudah.
Since we are dealing with the matter of the fence, our mishnah now seeks to clarify what qualifies as a fence in halachah: a partition that separates between the growing things - between the vineyard, for example, and an area in which one wishes to sow vegetables - and which forms an interruption, a separation and a boundary between the two, so that one may sow right up against the partition on both of its sides. What, then, is considered a fence?
What is a fence?
"shehu gavoha asarah tefachim" - a fence is considered a fence when it is ten tefachim high.
"v'charitz shehu amok asarah v'rachav arba'ah" - and likewise a trench, a ditch, that separates between the vineyard and other plants: it must be at least ten tefachim deep and four tefachim wide.
In summary: in this mishnah we learned the view of Rabbi Yehudah, that the definition of mechol hakerem does not apply next to a fence - there it is enough to set aside four amos - but rather between two separate vineyards, and there twelve amos are required according to Beis Hillel and sixteen according to Beis Shammai. We looked at the position of the first tanna, who disagrees, at the words of the Yerushalmi concerning vines that line up opposite one another, and finally at the measurements of a separating partition: a fence ten tefachim high, and a trench ten tefachim deep and four tefachim wide.