The Shulhan Aruch in Siman 10 discusses whether various garments are obligated in Sisit. One of the garments he deals with is a certain type of four-cornered cloak worn in Egypt and Spain, in which all four corners were worn in the front. The common custom was not to attach Sisit to this garment, and the Bet Yosef suggests two possible justifications. First, the garment was only used as an indication of distinction and did not have a functional purpose to warm the body. Second, the corners reached very low, and attaching Sisit would cause them to drag on the ground and become torn. In such a case the Torah did not require Sisit.
The Rama (Rabbi Moshe Isserles of Cracow, 1525-1572) in his Darkeh Moshe rejects these two approaches and offers a third explanation of his own. He says that a four-cornered garment is obligated in Sisit only when two of the corners are in the front and two are in the back. In this case, since all four corners are up front, the garment is exempt.
This principle has important ramifications for determining whether the today's classic collared shirt requires Sisit. The collared shirt actually has four corners: The two points of the collars and the two shirt tails at the bottom. According to the Rama's principle, the shirt is exempt because all four corners are in the front. However, the Bet Yosef did not bring this reasoning. According to his two explanations, the shirt would be obligated, since it is not worn specifically for distinction, nor do the corners hang low. Hacham Ben Sion discussed this issue and concludes that the custom of the Sepharadim is to rely on the Rama and not to put Sisit on shirts.
There is also a discussion whether the rabbinic frock, classically worn by Roshe Yeshiva, is obligated in Sisit. The frock has four corners, two in the front and two in the back, and would seemingly be obligated in Sisit according to the Rama. However, according to the Bet Yosef's two explanations, perhaps there is room to be lenient, since it is both a long garment and could also be considered a garment of distinction. To avoid any problems, the Mishna Berura (R. Yisrael Meir Kagan, 1839-1933, Poland) recommends that either one of the corners be rounded or insuring that the slits are not open more than half the length of the garment.
SUMMARY
Collared Shirts are not obligated in Sisit.
Rabbinic frocks would potentially be obligated in Sisit, unless one of the corners was rounded or the slits do not open more than half the length of the garment.
