(Mishnah): A Karachas HaKerem (empty patch) within a vineyard - Beis Shammai say that it should be 24 Amos (in order to allow planting other seeds there). Beis Hillel say 16 Amos.

A Mechol (strip of land on the edge, that surrounds) a vineyard - Beis Shammai say it should be 16 Amos; Beis Hillel say 12 Amos.

What is a 'patch of a vineyard'? A vineyard that has had its vines removed in the middle. If there aren't 16 Amos there, he shouldn't bring in another seed there.

If there are 16 Amos there, they give it work access and they may sow the rest of it.

What is a Mechol of a vineyard? It's the area between the vineyard and its fence. If there aren't 12 Amos there, he shouldn't bring in another seed there. If there are 12 Amos, they give the vineyard its work access and he may sow the rest of it (aside from the 4 Amos closest to the fence where people don't plant).

(R. Yehuda): That's merely called 'the vineyard fence'. So what is a Mechol (that requires a 12 Amah space)? The space between two vineyards.

It's considered a fence when it's 10 Tefachim tall. It's considered a trench when it's 10 Tefachim deep and 4 Tefachim wide.

(Gemara) (R. Yochanan): A Karachas HaKerem seems to be the same as a Kerem She'charav (in Perek 5 Halachah 1 - Menachos 63). (What's the difference between them?)

In a Karachas HaKerem the middle is removed; in a Kerem She'Charav, even the sides of the vineyard are removed.

(R. Yosi): When we say that a vineyard that's removed in the middle is like a Karachas HaKerem, it's specifically when the vineyard is large enough that on each side of it remains 2,2 and 1 vines (which is the minimum arrangement to be considered a vineyard), but if not, it is not considered a Karachas HaKerem.

Support (Baraisa): What is a Karachas HaKerem? A vineyard that is removed in the middle and there still remains a vineyard's amount on each side, whether on all four sides, three sides or one on each side.