Eruvin, Chapter 1, Mishnah 8. The Mishnah deals with a caravan of people camped in a valley, which has the status of a karmelis - it is not a public domain, since there is no public thoroughfare there, but neither is it a private domain - and those camped there are looking for a way to permit carrying. The solution the Mishnah offers: "Shayarah shechantah babik'ah - hikifuha bichlei behemah, metaltelin besochah" - a caravan that camped in a valley, if they surrounded it with animal equipment, they may carry within it. They surrounded the campsite with the implements used for the animals, saddles and the like, and once they have made a fence out of these implements, they are permitted to carry within that fence.
Two conditions for this fence:
"Uvilvad sheyihyeh gader gavoha asarah tefachim" - provided that the fence be ten tefachim high. Ten tefachim is the minimum measure needed for something to be considered a partition, a barrier that permits carrying.
"Velo yihyu peratzos yeseiros al habinyan" - the gaps must not be greater than the built-up portion. It is therefore permitted for there to be spaces between the implements, and there is no need for a completely solid partition, as long as most of the perimeter consists of the solid material - the implements - as opposed to the spaces between them.
In practice, even though it says that the gap may not exceed the built-up portion, the Gemara says that if they are equal - half and half - that too is sufficient.
The measure of a single gap:
The Mishnah continues and adds a further point: even if most of the camp is enclosed and only a minimal amount is open, there is a limit to the size of a single opening, even when it is in the minority. This is what the Mishnah says: "Kol pirtzah shehi ke'eser amos - muteres, mipnei shehi kepesach" - any gap that is ten amos or less is permitted, because it is considered like a doorway. Any opening of ten amos or less is viewed by us as a doorway in a wall, and is therefore permitted. But "yoser mikan - asur" - more than this is forbidden: an opening larger than ten amos is so wide that we cannot view it as a doorway in a wall, but only as a breach in the wall, and that is not enough.
Tzuras hapesach:
However, if they make a tzuras hapesach there, the form of a doorway, as was explained in the first Mishnah of the chapter - two reeds at the sides and a reed passing across the top of them - it is permitted to carry there even if the opening is wider than ten amos.
In summary: In this Mishnah we learned how a caravan camped in a valley that has the status of a karmelis makes its place fit for carrying by surrounding it with animal equipment, subject to two conditions: a fence ten tefachim high, and that the gaps not exceed the built-up portion (and if they are equal, it is permitted). We also examined the measure of a single gap: up to ten amos it is considered a doorway and is permitted, more than this it is considered a breach and is forbidden - unless they made a tzuras hapesach in it.