cellio | daf bit: Eruvin 76

As previously learned (at some length), the residents of houses around a courtyard can make an eiruv so that they can all carry to and from that courtyard. The courtyard must be delineated by walls and valid openings. Today's daf discusses two adjacent courtyards. The mishna teaches that if in the wall (fence?) between the two there is a window at least four by four handbreadths, and with its lowest portion no more than ten handbreadths from the ground, then this is a valid opening and the residents around both can make either one combined eiruv or two separate ones, their choice. If the window is too small or too high, it does not constitute a valid opening and each side must form its own eiruv. In the g'mara R. Hachman argues that the height restriction applies only between courtyards, not to windows between upper floors of buildings (row-houses?), but the window still must be at least four by four handbreadths no matter where it is. (76a mishna, 76b g'mara)