How To Do The Partial Tuck RIGHT | A Shirt Tucking Tutorial (alpha m.)
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Some people like their T-shirts and sweatshirts tucked, most prefer them untucked. But here is a third option: the partial tuck. This gives a more casual and cool look and lets you show off that killer new belt you so madly want to show. Here is how to do the perfect trick. But, as he says, don't try this with a button-up shirt, it always looks messy and awkward.

And here is another informative video about how long can a T-shirt or a button-up shirt be when untucked, so you look cool and not a disaster: SHIRT LENGTH WHEN UNTUCKED.