In a recent order a customer intended to select t-neck but accidentally selected round neck. He didn't notice his mistake until the order had already been delivered to him.
I don't know if was any consolation, but told him that personally I'm in favor of round neck tzitzit (i.e. tallit katan) and switched to it about a year ago.
For a tallit katan to be kosher (i.e. to fulfill the mitzvah of tzitzit) it has to have a minimum size requirement. To meet that requirement isn't always such a simple matter.
One solution is to create a slit along the front, so that you can have a relatively small neck opening (without the slit you wouldn't be able to put it on). If you do that, and you want to measure the beged from top to bottom to ensure it meets the minimum size requirement, how do you measure it? From the bottom of the slit or the bottom of the neck opening.
Most people will tell you that you can measure from the bottom of the slit, and consider it as if it's closed, just like a button-up shirt is considered whole in front. But some poskim do not agree, and I think I have some very strong practical evidence to back that.
After a short time of wear, just about any tallit katan with a slit in front starts to lay down permanently, like lapels, and form a deep crease. So if you now have a tallit katan with "lapels," why should you count that as if it's closed?
