“ The ropes themselves must be a tefach and a little bit - the whole thing must be ten tefachim. Explanation The talmudic principle of lavud states that any gap of 3 tefachim or less is not considered a gap at all. This mishnah uses this principle to allow someone to build an eruv (for which the borders must be 10 tefachim high) , with just three ropes which are seperated by 3 tefachim or less. To make the ten tefach total, the ropes themselves must have a total thickness of a tefach. ”