To be obligated in Tzitzis, the garment must be considered as having four corners. According to the Alter Rebbe in Shulchan Aruch, if the garment is sewn down the majority of the length of the sides or tied together with straps, it is no longer considered a four-cornered garment and is not obligated in Tzitzis. If only part of the garment is sewn together and the majority remains open, the garment is obligated in Tzitzis. Because of Maris Ayin, it must be clearly noticeable that the majority is open.
If the sides are hooked together or held together with snaps that can easily be undone, the garment technically remains obligated in Tzitzis even if the snaps go down the entire length. Nonetheless, because of Maris Ayin the snaps or hooks should not be placed lower than the half-way point of the Tzitzis.
In the Siddur, the Alter Rebbe is more stringent, writing that if one wants to fulfil the Mitzvah of wearing Tzitzis all day (by wearing a Tallis Katan) and be Yotzai according to all opinions, the Tallis Katan should be open completely on both sides and should not even be held closed with hooks or snaps, even if the majority remains open.
When the Alter Rebbe rules differently in the Shulchan Aruch and the Siddur, Chabad Chassidim follow the Siddur, being the later ruling. Therefore it is best not to wear singlet Tzitzis (unless nothing else is available).
