Raba sent to Abaye by the hand of R. Shemaiah b. Ze'ira [the following message]: ‘Do we not find a partition to be the cause of a prohibition?
Surprising, since you’d think the point of a partition would be to create, for example, an eruv which loosens the restrictions of carrying on Shabbat. We look here to the example of kilayin, the command not to sow with mixed seed.
Was it not in fact taught: ‘partitions in a vineyard may be either the cause of a relaxation of the law or one of a restriction of it.’ In what manner? If the plantation of a vineyard stretched to the very foundation of a fence one may sow from the very foundations of that fence and beyond it; whereas in the absence of a partition one may sow only at a distance of four cubits (from the vineyard); and this is an example of a partition in a vineyard that is the cause of a legal relaxation. In what manner are they a cause of legal restriction? If a vineyard was removed eleven cubits from a wall no seed may be sown in the intervening space; whereas in the absence of a wall one may sow at a distance of four cubits; (from the vineyard) and this is an example of a partition in a vineyard that is the cause of a legal restriction?’
Seeing that the partition can be used for both purposes – or can affect the law’s extension or relaxation – its use has to carefully considered. A partition can sometimes be actual, sometimes virtual – as in the case of side-posts:
If an exedra that had side-posts was covered with boughs, it is valid as a sukkah
The two walls extending virtually to create a third necessary for a sukkah.