Kilayim, Chapter 6, Mishnah 7. The Mishnah teaches a further halachah in the laws of an aris (a trellis of vines): an aris that extends from a wall.
The text of the Mishnah:
"Aris sheyatza min hakosel mitoch hakeren v'chalah" - an aris that grows out of a wall, that is, the fence upon which the vines lean, and it extends from the corner, from the edge - and in practice from both corners - and then comes to an end.
Explanation of the case:
The Rav (Bartenura) and the Rash explain that this refers to a case in which this bed contains a total of five vines, which is the minimum measure for a bed, except that they do not grow together:
Three vines at one end of the wall.
Two vines at the other end.
And in the middle - empty space.
The halachah:
Since there are not five vines together here, this combination does not have the status of a full bed, and therefore "nosnin lo avodaso" - we give it its working space, six tefachim, and that is all that is required, "v'zorea es hamosar" - and the rest may be sown.
The opinion of Rabbi Yosi:
"Rabbi Yosi omer: im ein sham arba amos, lo yavi zera l'sham" - if the wall does not have four amos in total, one may not sow there, even though this is not a full aris. The total area must be at least four amos.
This principle was already stated in the halachah in Chapter 5: there Rabbi Yosi said that one who sows in a wine press or in a nekah - a hollow, a depression - must have at least four amos in the place, so that it be possible at all to separate six tefachim. The same applies here: a total of four amos is required in the wall, and only then can one separate six tefachim from each of the vines and sow there.
In summary: in this Mishnah we learned about an aris that extends from a wall out of the corner and comes to an end - only five vines, three at one end and two at the other, which do not have the status of a full bed; therefore they are given a working space of six tefachim and the rest may be sown. In the opinion of Rabbi Yosi, consistent with his view in Chapter 5 regarding a wine press and a hollow, a total of four amos is required in the place, and without them one may not bring seed there at all.