Are undershirt tzitzit and t-shirt tzitzit kosher? A year or two ago I came upon a batch of t-shirt tzitzit that had a halachic problem. Since then I have learned to be very wary when I buy from wholesalers, because I’ve discovered that the problem can come up unexpectedly, even with highly reputable manufacturers.
For a garment to require tzitzit, it has to be more open than closed (see Shulchan Aruch O.C. 10, 7). So you lay it flat on a table and measure from the corner up along the side to the stitch line that rests on your shoulder. Let’s say it’s 31 inches. (Ignore the sleeve, i.e. pretend as if it’s closed.) You then have to make sure that the sides are open from the corner at least 16 inches up before the beginning of the stitches that close it along the sides.
There’s also a quick way to check it if you don’t have a tape measure handy: Press your finger right at that point where the open section meets the closed section. With your other hand, grasp a corner and fold it straight up toward the shoulder. If the corner goes past the top of the tzitzit garment, you’re fine.
The vast majority of tallit katan makers are well aware of this requirement, but when they commission a large production run, sometimes a certain size may come sewn improperly so that you don’t have a majority of the sides open.
I have noticed that this can happen occasionally, even with upstanding manufacturers (I won’t mention any names). What’s always surprised me is how this gets past the tzitzit tie-ers. Most of the undershirt tzitzit have machine-spun tzitzit tied on in Israel by the manufacturer. They tie in very large quantities (e.g. they may send out a batch of a few thousand at a time), but how can an erlich worker tie tzitzit for a few hours a day and not notice the problem? Most of these products have a hechsher, but note that the hechsher invariably applies to neither the garment nor the tying, only the tzitzit strings.