Kilayim, Chapter 4, Mishnah 8. The Mishnah opens: "HaNotei'a shtei shurot, im ein beineihen shemoneh amot - lo yavi zera lesham" - one who plants two rows of vines with less than eight amos between them may not bring seed of other species there.
How many vines are there in these two rows?
The Rav explains that we are speaking of two vines in each row, four in all. Now, ordinarily the minimum measure of a vineyard requires three vines in each row, or alternatively three in one row and two in the other, while two and two are not enough.
The Rav's approach:
The Rav understands that everything said about the minimum measure relates to the distance one must keep from the vineyard on the outside: it does not have the status of a vineyard until it contains six vines, or at least five, and from the moment it has the status of a vineyard one must keep four amos away from it. Here, however, we are dealing with what is potentially inside the vineyard, and for this matter - the prohibition against sowing within it - it is considered a vineyard even with two rows of two, unless there are eight amos between them. If there are eight amos between them, those vines are treated as individual vines, and a distance of six tefachim is enough in order to sow, like the law of any single vine.
The approach of the other Rishonim:
Other Rishonim disagree and say that we are dealing with a case in which there are three vines in each row, that is, an ordinary vineyard. But if there are eight amos between them, since he planted them at a distance of eight amos from the outset, it does not receive the status of a vineyard at all.
"Hayu shalosh":
The Mishnah continues: "Hayu shalosh, im ein bein shurah lachaveiro shesh esreh amah - lo yavi zera lesham" - if there were three rows, eight amos are not enough, for at this point the planting has the status of a large vineyard, a broad and spacious vineyard, and therefore sixteen amos are required, like the law of the karachas hakerem - the bald spot within the vineyard.
And there is an exception here: since we have only three rows, if the middle row was destroyed and only two rows remain, this is not treated as a karachas hakerem requiring sixteen amos; rather, eight amos are enough, like the law of two rows alone.
The opinion of Rabbi Eliezer ben Yaakov:
Rabbi Eliezer ben Yaakov, in the name of Chananyah ben Chachinai, disagrees and says: "Afilu chorvah ha'emtza'is, ein bein shurah lachaveiro shesh esreh amah - lo yavi zera lesham" - even though there were three rows and the middle one was destroyed, if there are not sixteen amos there, one may not sow there.
And he continues: "She'ilu mitechilah neta'an - harei zeh mutar bishmoneh amos" - had the two remaining rows been planted this way from the outset, it would have been permitted with only eight amos, according to the ratio established at the beginning of the Mishnah. But since there were three rows here and the planting received the status of a large vineyard, even after the middle row was destroyed it retains the status of a karachas hakerem, and it requires sixteen amos in order to sow there. According to Rabbi Eliezer ben Yaakov, once it received that status, the status is never removed from it, even after it was destroyed.
In summary: in this Mishnah we learned the required distances between rows of vines in order to permit sowing other species between them: two rows - eight amos, and three rows - sixteen amos, because of their status as a large vineyard and the law of karachas hakerem. The Rishonim disagree about how many vines there are in each row and about whether the distance negates the status of a vineyard from the outset, and the Tannaim disagree about whether the status of karachas hakerem is retained even after the middle row was destroyed.