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The four corners of Tzitzis: 
All four corners of Tzitzis are considered a single Mitzvah, and therefore so long as a Tallis does not have all four Tzitzis attached to it, it is not Kosher.
 13/1; Michaber 13/1; Mishneh Menachos 28a
Other opinions: Rebbe Yishmael in Mishneh Menachos 28a is of the opinion that each of the four corners is a separate Mitzvah, and one hence fulfills four positive commands in wearing Tzitzis. We rule like Chochamim. See However P”M 13 M”Z 5 that even according to Rebbe Yishmael one may not wear a four cornered garment without Tzitzis on each corner. However see Bach there.