Mishnah/Seder Moed/Tractate Eruvin/Chapter 1/9
This mishnah continues the discussion of the previous case where a travelling caravan had camped in a remote area for shabbat. It discusses another method of that caravan creating an eruv with a minimum of equipment.
- מקיפין שלושה חבלים, זה למעלה מזה זה למעלה מזה--ובלבד שלא יהא בין חבל לחברו שלושה טפחים;
- שיעור חבלים--עוביין יתר על טפח, כדי שיהא הכול עשרה טפחים.
- Surround an area with three ropes, one on top of the other, and there isn't [a gap of] 3 tefachim between one rope and another;
- The ropes themselves must be a tefach and a little bit - the whole thing must be ten tefachim.
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.