The well-known American clothes retailer Abercrombie and Fitch produce a range of tops for men called ‘Muscle’ fit.
These t-shirts have a slight upwards incision towards the bottom of the tshirt so that technically it there are four corners. (If you need a visual, please see: http://www.bridgat.com/files/Abercrombie_Deer_Brook_Men_Polo.jpg).
Are such t-shirts chayav in tzitzit (if so, they obviously shouldn’t be worn as people are extremely unlikely to tie their own tzittzit to them!)?
From the picture, it seems clear that this item of clothing is not obligated in the mitzvah of tzitzis.
The reason for this is that the great majority of the top is “closed,” and only the very bottom in “open,” forming the corners at the bottom of the shirt.
The obligation of tzitzis depends on the majority of the garment being “open,” and therefore in this case there’s no problem of wearing the shirt without tzitzis.