May one attach two sides of his clothing using a safety pin?[1]

Regarding making it into a double stitch: Some Poskim[2] rule that this should not be done. Others[3] rule that so long as it is not meant to be placed there for permanent use [as is usually the case] it is allowed.[4] The Rebbe[5] defends the practice of making even double and triple stitches for use on Shabbos, however he forbids making more than three stitches.

A single stitch:[6]: According to all[7] one may enter the pin into a single stitch, which means that he enters it on one side and has it lift out on the other side.

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[1] Piskeiy Teshuvos 340:22

[2] Mishneh Berurah 340:27, in name of Karban Nisanel; Minchas Yitzchak 2:19; Sheivet Haleivi 4:35; Poseik who wrote to Rebbe about forbidding pinning clothing on Shabbos.

[3] Sharreiy Teshuvah 340:3; Chazon Ish Supplements to Orach Chayim 156; Igros Moshe 4:84; Tzitz Eliezer 13:43

[4] The reason for this leniency is because: a) The concept of sewing does not apply to metal; b) It is not made to last [of which is only

Rabbinically forbidden] c) Is being done for a Shabbos use. It is thus permitted as by a Rabbinical prohibition within a Rabbinical

prohibition we are lenient.

for a Shabbos use. [Rebbe in Shaareiy Halacha Uminhag 2:164] 

[5] Shaareiy Halacha Uminhag 2:164

[6] So is implied from M”B 340:27, and so rules Minchas Yitzchak ibid, Rebbe in Shaareiy Halacha Uminhag Ibid and

the other stringent opinions regarding making two stitches, that by one stitch it is permitted according to all.

[7] So writes Rebbe in Shaareiy Halacha Uminhag ibid