As we have already learned, the general assumption of the Talmud is that everything that is permanently attached to a building is included in a standard sale, while removable objects are not.
According to the Mishna, when a person sells an olive press – a beit habad – it includes the yam, the memel and the betulot, but not the avirim, the galgal or the kora. When the seller specified that he was selling the entire contents of the beit habad, then all are included. According to Rabbi Eliezer, the kora is included in all situations.
Although the Gemara asks for definitions of all of these terms, their actual meanings remain unclear to us today. The rishonim also interpret them in different ways, based on a variety of sources and traditions that they used and what was commonly found in their own communities. It appears that the process of extracting oil from olives was different in different places, and even the descriptions found in the Mishna and baraitot may refer to different methods.
The beit habad was a structure that included permanent parts, like the yam, a round structure that held the olives, betulot, wooden beams that supported the kora, and the memel, which was a stone or other object that broke the olives up.
It also included parts that were removable, like the avirim, which were placed on the olives to weigh them down and help extract the oil, the galgal which was a weight connected to the kora that helped push it up and down, and the kora itself which was a beam that pressed down on the olives.
Rabbi Eliezer’s position is understood by the Gemara as placing the kora – the central beam – as the defining structure of the beit habad, and as such, it must be included in the sale even if it is not permanently connected to the building itself.