Eruvin 10b: “The open space on both sides [of the structure] comes and cancels it.”
The Gemara here introduces the rule of “asi avira d’hay gisa u’d’hay gisa u’mevatel lei” (Eruvin 10b; Shulchan Aruch, 363:34). This principle translates as: “The open space on both sides [of the structure] comes and cancels it.” The simplest application of this principle is a case where on one side of a walled enclosure there is an open space of, say, six amos, then a standing structure of five amos, then again an open space of six amos. In this case, the five amos long structure is negated by the surrounding open space and the open spaces combine to create a pirtza of more than seventeen amos. This principle applies even if the open space on one side is equal in length to the structure, as long as the space on the other side is somewhat longer. There are many complex variations of this problem, especially when an eruv has a series of contiguous structures and open spaces. See the details in Hilchos Eruvin 6:3-7 and Nesivos Shabbos 14:7.
Accordingly, Rashba here notes that the principle of omed merubeh al ha'parutz that allows us to treat a side of an enclosure that is mostly fenced as if it is completely fenced is only operative if the fences begin at the corners, or are structured in some other way that prevents segments of the fence from being cancelled by surrounding airspace.