Eruvin 12: 
Terminology: Pas/pasim. R. Eliezer visits his student, R. Yosi ben Preida, and the topic of conversation is the student's need for a second lechi on his alleyway. But how big of a need is it? Beit Hillel and Beit Shammai debate the need for lechi and koreh (is that both or either?!). And their debate answers R. Yosi ben Pereira. Now, another case - with only one board (pas) in a courtyard that has a breach, in this case, by the sea, and the need is to close it up. Do you need 2 pasim? Plus: A new case on the shape of the alleyway or courtyard. How wide do those pasim need to be? Length vs. width? How really are the different from a lechi?