"One case is where it is made like a peg (sikhta), and the other is where it is made like an istivra. Rashi explains that a sikhta is invalid because it is inserted into the doorpost like a bolt that is stuck into a wall, whereas an istivira—the place where the thigh and leg are joined—it stands upright, and thus the halakah is that it is valid. According to this explanation, whether it's standing or whether is is lying down, they are both called 'like a bolt.' This is difficult for Rabbenu Tam, because an upright position is not considered respectful. In Bava Batra (101a) it is called a 'donkey burial' when they bury one upright. Likewise, the Torah scroll and the Tablets in the Ark were laid horizontally rather than standing upright, as is implied in the first section of Bava Batra 14a, even though they could have been placed upright, since the Ark's height equaled its width. Furthermore, it is said later (in our gemora): 'If one made for it a frame of reeds, he cuts a tube and places it [there].' If the mezuzah were to stand upright, he could simply place it into one of the reeds of the frame. Therefore Rabbenu Tam explains that 'like a peg' means upright, like the stakes of the Tabernacle that were driven into the ground, and this is invalid. 'Like an istivra,' which is placed across the leg horizontally (called cabilla in Old French), is valid. However, in the chapter of chalitza Yevamot 103a, where it says 'this istivra reaches down to the ground,' it is clear that the term refers to the bone extending from the thigh to the foot, not to a cabilla. However, the statement in the Jerusalem Talmud (Megillah chapter 2) that 'the word Shema must face the entrance' is not proof for Rashi's interpretation, because according to Rabbenu Tam it can be explained as follows: the last line is toward the public domain and the first line toward the inside, and he turns the word 'Shema' toward the airspace of the doorway. Rabbenu Tam also used to place the passages of the tefillin horizontally rather than upright. However, the statement below (Menachot 34b), 'If he wrote it on one piece of parchment and placed it in four compartments, he fulfilled his obligation,' presents no difficulty, because that too must be explained even if the parchments stand upright. A difficulty for Rabbenu Tam arises from a Tosefta in Kelim: 'If one cut a tube, placed a mezuzah in it, and afterward put it into the wall, even if not according to its normal manner of use, it is susceptible to impurity. If he put it into the wall first and afterward fixed the mezuzah in it, then if it is according to its normal manner of use it is susceptible to impurity, and if not according to its normal manner of use it is pure. This implies that the tube is closed on one side. According to Rabbenu Tam, who says that a mezuzah placed like a bolt is invalid but when laid horizontally is valid, what does 'according to its normal manner of use' mean? It may be answered that the tube is open lengthwise while both ends are closed, and 'not according to its normal manner of use' means that it was turned upside down over its opening. This requires further investigation: The statement, 'All the mezuzot in Rabbi's house were made like a bolt,' implies that other mezuzot generally were not made that way. Therefore it appears, whether according to Rashi or according to Rabbenu Tam, that the valid position described as 'like an istivra' is diagonal—neither fully horizontal nor fully upright. Alternatively, it may be like the shape of the Hebrew letter nun, partly horizontal and partly upright. Rashi also explained it this way in another interpretation. Other mezuzot were not made in this fashion. According to Rabbenu Tam they were horizontal; according to Rashi they were upright. Rabbenu Tam says: Go and learn from common practice. Every day, when the Torah scroll stands on the bimah, everyone stands; and when the prayer leader lays it down, everyone sits."