Eruvin, Chapter 7, Mishnah 4. In the previous mishnah we learned that a ditch can serve as a partition separating two courtyards. Our mishnah continues with this idea and examines the status of a ditch over which a board has been laid.
"Nasan alav neser":
A board four handbreadths wide that was laid across the ditch becomes like an opening between the two courtyards, since it allows free passage from one side of the ditch to the other. Even though the ditch separated the courtyards, now something like an opening has been created between them.
"V'chein shtei gezuztra'os zu k'neged zu":
The mishnah teaches a similar law regarding two balconies standing opposite one another: if a plank four handbreadths wide was placed between them, so that a person can walk across it from one balcony to the other, their status is like that of two courtyards with an opening between them.
In both of these cases the choice is theirs, exactly as in the case of two courtyards with an opening between them:
"Me'arvin shnayim" - each one may make a separate eruv for itself.
"V'im ratzu me'arvin echad" - and if they wish, they may join together in a single shared eruv.
"Pachos mikan":
But if the plank is less than four handbreadths wide - a person cannot walk across it easily, for he is no acrobat who walks on a thin rope. Therefore it is not viewed as an opening, and the ditch (or the gap between the balconies) remains a divider. Consequently their law is: "Me'arvin shnayim v'ein me'arvin echad" - each of the courtyards must make its own separate eruv, and they cannot join in one eruv together.
In summary: In this mishnah we learned that a board four handbreadths wide laid across the ditch, and likewise a plank of that width laid between two balconies that face one another, are considered an opening that allows passage - and therefore "they make two eruvin, and if they wish they make one." Less than four handbreadths, there is no way to pass, the separation remains in place, and "they make two eruvin and they may not make one."