How is one who is left-handed to wear Tefillin of one who is right-handed [or vice versa], if his own Tefillin is not available?
Introduction: The only difference between the Tefillin worn by one who is left-handed versus one who was right-handed, is the positioning of the knot of the Yud which is adjacent to the Ketzitza of the hand Tefillin. It is customary for this knot to face inwards towards the heart, and hence this knot is adjacent to different sides of the Ketzitza by a righty versus a lefty’s Tefillin. Accordingly, a lefty who wears the Tefillin of a righty on his right arm ends up having the knot face away from his heart, and the question is thus asked as to what he is to do. The same question applies vice versa, if a right-handed person only has a left-handed person’s Tefillin available.
The law: If the only available Tefillin for a lefty is one of a person who is right handed, or vice versa, then some Poskim rule that one is to undo the knot of the Yud and retie it on the other side, so that when he places it on, the knot faces inwards. If this is not possible, then one is to place the Tefillin on his right arm backwards, having its backside [i.e. Ma’abarta] be positioned in front facing towards the elbow, in order so the knot be facing inwards. There is no Halachic invalidation in wearing the Tefillin in such a way. Practically, many even initially follow this second option, especially if one does not know how to untie and redo the knot, or if the owner may not want him to do so, or if one is short on time [such as on Mivtzaim]. [When wearing the Tefillin in such a way one must be careful that the Tefillin be properly wrapped to the bicep, without it folding over. However, one should not follow the option of placing the Tefillin on as normal with the Ma’abarta facing the back, thus causing the knot to be on the outer side. However, Bedieved, if the Tefillin was worn in the normal way with the Yud of the Tefillin facing outwards, he nevertheless fulfills his obligation.]
 See Piskeiy Teshuvos 27:5
 Admur 27:4; Michaber 27:2
 Nachlas Shiva 41; Kitzur SHU”A 10:7; Biur Halacha 27:3 “Haminhag Hanachon” in name of Chayeh Adam
Other opinions: Some Poskim rule that one is not to switch the knot of the Yud and is rather to always put the Tefillin on in the reverse order, as explained next. [Shvus Yaakov 1:3; Omitted from majority of Poskim in next footnote]
 Shvus Yaakov 1:3; Beir Heiytiv 27:10; Machatzis Hashekel 27:3; Yeshuos Yaakov 27:4; Artzos Hachaim 27:3; Aruch Hashulchan 27:17; Kaf Hachaim 27:11 and 27, 31; Kitzur SHU”A ibid; Biur Halacha 27:3 “Haminhag Hanachon”; Yagdil Torah N.Y. 6:51; Hiskashrus 430:16; Shevach Yakar p. 73; Dinei Iter 5:6; See Nesivim Bisidei Hashlichus p. 45
 As having the Ma’abarta face towards the shoulders is a mere good custom and not a matter required from the letter of the law. [See Admur 27:6; Michaber 27:3]
 See Piskeiy Teshuvos ibid footnote 27
 The reason: Although both the wearing of the Tefillin in a way that it’s knot faces the heart and that it’s Mabarta faces the upper part of the shoulder is a mere good custom and not obligatory [See Admur 27:4; Michaber 27:2 regarding the Shin facing inwards and Admur 27:6; Michaber 27:3 regarding the Mabarta facing the shoulders], nonetheless one with the Tefillin of a person who wears it on the opposite arm should always choose to wear it in a way that the Mabarta is towards the elbow rather than have the knot face away from the heart. The reason for this is because the custom of wearing the knot of the Yud towards one’s heart is of greater importance than having the Mabarta facing the shoulder. [Se Biur Halacha ibid in name of Gra 27:7 and 9 that having the Yud face the heart is a Talmudic requirement as opposed to a mere custom]