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Summary

3 tefachim of empty space disqualifies a sukkah. (This refers to a sukkah of only 3 walls. If a sukkah has 4 walls and is split in middle by an empty space, it becomes two kosher sukkahs with 3 walls each).

Abayye: In a large sukkah the disqualification caused by 3 empty tefachim can be reversed by a wooden stick or an iron rod. In a small sukkah [precisely 7x7 tefachim] the disqualification caused by 3 empty tefachim can be reversed only by a wooden stick, not by an iron rod. (Because here it is lacking kosher schach).