Kilayim, Chapter 4, Mishnah 6. In the previous mishnah we learned that according to Beis Hillel the minimum measure of a vineyard is two rows of three vines each - six vines in all. Our mishnah teaches that there is also an abbreviated version of a vineyard, and this is its form.
The formation that counts as a vineyard:
"Hanotei'a shtayim keneged shtayim ve'achas yotza zanav - harei zeh kerem" - one who plants two vines opposite one another, and in one of the rows plants a third vine that extends out like a tail. That is: one row has two vines and the second row has three. This formation is considered a vineyard, even though it is smaller than the usual minimum measure (the diagram in the Kehati commentary is helpful here).
The formations that do not count as a vineyard:
"Shtayim keneged shtayim ve'achas beinsayim" - three vines planted in one row, with two vines planted opposite the two outer ones, so that the additional vine stands between them.
"O shtayim keneged shtayim ve'achas ba'emtza" - two vines opposite two vines, with the fifth vine planted in the middle, inside the square formed between the four of them.
"Eino kerem ad sheyihyu shtayim keneged shtayim ve'achas yotza zanav" - these two formations are not considered a vineyard. Only the arrangement in which two vines are planted opposite two vines, with the third extending out like a tail from one of the rows, counts as the abbreviated form of a vineyard.
In summary: in this mishnah we learned that besides the usual measure of six vines in two rows, there is also an abbreviated formation that counts as a vineyard - two opposite two with one extending out like a tail. But two opposite two with one in between, or two opposite two with one in the middle, are not considered a vineyard.