The dimensions of a korah
 The requirements for the dimensions of a korah are as follows: 1) Technically the korah must be long enough to stretch across the entire opening of the alleyway. However, based on the rules of lavud it is sufficient if it reaches within three tefachim of the edge of the wall. 2) The korah must be at least a tefach wide. 3) The korah must be strong enough that it would be able to support a brick that is three tefachim long and one and one half tefachim wide . The reason for this is so that it be noticeable to the onlooker that it is a permanent part of the alleyway, and not just a temporary addition. [ שו"ע שסג, יז, משנ"ב נט, וביה"ל ד"ה חזקה]
 A circular korah
 A circular korah must have a circumference of at least three tefachim. A korah that is at least four tefachim wide does not need to be strong enough to support a brick (see above). The reason for this is that a korah that it is four tefahim wide is noticeable even if it is not overly sturdy. Many Rishonim say that the pegs in the wall that support the korah must also be strong enough to support the korah and the brick. [ שו"ע שסג, יז-יט, משנ"ב סא-סב, שעה"צ מד, וביה"ל ד"ה חזקה; וראה משנ"ב ס; ביאורים ומוספים דרשו50,]
 Calculating the width of a circle
 Chazal state that a circle with a circumference of three tefachim is at least a tefach in diameter. This is why a round korah must have a circumference of at least three tefachim. The Rishonim point out that this is not an exact calculation as the exact measurement of a circle that would contain a tefach across its width would be 3.14 tefachim. Nonetheless, a three tefach round korah is acceptable as Chazal preferred to issue a simple even calculation so as not to confuse people with a more complicated formula. [ שו"ע שסג, יט; ביאורים ומוספים דרשו, 52]
 A korah may be placed over the opening of an alleyway in order to permit carrying within its confines. A koreh must be built for the purpose of allowing carrying within the alleyway. A lechi that is less than four amos long may be built as a continuation of one of the walls of the alleyway although it is sticking out into the street. However, the lechi must be thinner than the wall in order that it should be noticeable that it is a lechi not just a continuation of the wall.
 Combining two korahs How close must the two korahs be? Creating a korah that is four tefachim wide
 PLEASE NOTE: The information in this email is for learning purposes only. Please review the Mishna Berura and Biurim U'Musafim before making a halachic decision. Hebrew words are occasionally transliterated to enable a smoother reading of the text. Common Ashkenazi pronunciation is generally used in these cases.
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