What is the definition of "Sha'atnez"?
Refer to Vayikra 19:19:4:1-3 & 19:19:4:2. 1
Bechoros, 17a: Just as "Pishtan" is pure flax, so too, must "Tzemer" be pure wool - to preclude from Malkos the wool of a 'Nidmeh' 2 (a lamb whose mother or mother's mother is a goat). 3
Rashi (on the Beraisa of Rebbi Yishmael): "Sha'atnez" is a K'lal' and "Tzemer u'Pishtim", a P'rat, which precludes everything other than the P'rat (wool and linen).
By the same token, the Gemara (Ibid.) disqualifies the wool of a Nidmeh from being used for Techeiles.
See Torah Temimah, note 112 who discusses whether it is subject to an Isur d'Oraysa, or merely to an Isur de'Rabanan.
What does "Lo Silbash Sha'atnez" entail?
Targum Yonasan: It entails a. wearing a garment made of wool and linen; b. warming oneself with it by merely placing one's arm in the sleeve. 1
See Peirush Yonasan.
What is the significance of the comparison of Tzemer to Pishtim?
According to R. Yoshiyah, who says in Sanhedrin, 66a that a 'Vav' is inclusive, why does the Torah need to insert the word "Tzemer u'Pishtim Yachdav "?
Rashi (in Shabbos, 54a) and Targum Onkelos: To confine the Isur Sha'atnez to where the wool and linen are (firmly) joined 1 (and will not fall apart ? Rashi).
Moshav Zekenim #2 (in Pasuk 10): To preclude where the wool and linen are joined by only one stitch. 2
Yevamos, 5b: To teach us that one is only Chayav on Kil'ayim if the thread that joins the wool and the linen enters the garment at least twice. 3
Moshav Zekenim #1 (citing R"S): To preclude wearing two garments one on top of the other, which is permitted - . And by the same token, in the previous Pasuk, it comes to preclude attaching the two animals to different plows simultaneously, either one after the other, or with one ox going in one direction and a donkey in another direction in the same field (Moshav Zekenim).
See Torah Temimah, note 115. Moshav Zekenim (in Kedoshim Vayikra, 19:19): In fact, even two stitches only join [for Kil'ayim] if they are tied or woven. Mar Zutra (in Nidah 61b) permitted if a linen thread is sewn into a woolen garment, and one removed it, but is unsure if all of it came out, since it is a Safek mi'de'Rabanan.
See Torah Temimah, note 115, as to whether the two ends also need to be tied.
Why did the Torah forbid wearing Sha'atnez?
Refer to Vayikra 19:19:3:3-7 and notes.
Why does the Torah omit the word "Beged", which it inserted in Kedoshim Vayikra,19:19)?
Refer to Vayikra 19:19:154:1-5.
Having written in Kedoshim Vayikra, 19:19 "Lo Ya'aleh alecha", why does the Torah write here "Lo Silbash"?
Refer to Vayikra 19:19:155:1-3 and note.
Yevamos, 4b: In order to preclude sellers of garments who wear them, not for warmth, but to show their potential customers what the garment looks like - and "Lo Yilbash" implies that one derives intrinsic benefit from the garment.
