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Though there are no hard and fast rules when it comes to fashion, the vast majority of dress shirts are designed to worn tucked-in. Because of this, if you want to look as good as possible, you'll usually want to have your dress shirt tucked in with one of the methods above. While there are plenty of casual situations where it's just fine to wear an unbuttoned, un-tucked shirt with a tee underneath, it's hard to look better with this approach than you would by simply tucking your shirt in. ’’Always’’ tuck in dress shirts that hang past your hips. In these cases, the extra material can make your shirt look a bit like a flowing nightgown or even a dress, which will rarely be the sort of impression you'll want to make. Just as most dress shirts are made to be worn tucked, most polos and tees are meant to be worn un-tucked. When properly-fitted, these sorts of shirts should hang right up against your belt or the waist of your pants. You can tell this by looking at the way the bottom of a polo or tee differs from the bottom of a dress shirt — most will have a flat bottom edge, rather than long shirttails in front and in back. The exception here is when you have to wear an exceptionally-long tee or polo. In these cases, tucking the extra material in usually makes you look a little better. You can tuck in normal-length polos and tees as well, but this can sometimes give them an overly-tight appearance. When you're wearing a dress shirt, there are certain situations in which it's always advisable to tuck your shirt in. For instance, leaving your shirt un-tucked at many formal events or celebrations can be seen as a breach of etiquette bordering on disrespect. Below are just a few examples of the types of situations where you'll always want to have your shirt tucked in:  Weddings Graduations Religious ceremonies Funerals Court appearances In the business world, some situations will almost always call for a tucked-in shirt. Some of these situations are unique to certain jobs that demand formal behavior, but some, like job interviews, are things that almost everyone will encounter. Below are just a few examples of when you may need to tuck in your shirt:  Job interviews Meetings with new or important clients Meeting with strangers Serious work events (layoffs, new hires, etc.) Note that for many jobs, normal working days require tucked-in shirts or even suits. It's worth noting that some events which aren't exceptionally formal and aren't connected to work can still demand a tucked-in shirt. In these cases, an un-tucked shirt may not be seen as disrespectful, but may be a bad idea all the same. In these situations, you may want to look as nice as possible either to make yourself attractive or to show that you're taking things seriously. Here are just a few ideas for when you may want to keep your shirt tucked in:  Visits to fancy nightclubs or restaurants First dates "Serious" parties, especially when you don't know many people attending Art exhibitions and sit-down concerts It's important to remember that you definitely don't always have to tuck your shirt in. For instance, if you're staying in for the night, heading to a close friend's house, or just dining at a casual restaurant, there's no need to tuck in your shirt (or, truthfully, even to wear a dress shirt in the first place). Casual hangouts and other events where you won't be judged on the way you look have no need for tucked-in shirts, so unless you just want to look great 24/7, you can usually skip them.

Summary:
In general, tuck in button-up dress shirts. In general, leave polos and tees untucked. Always tuck in your shirt for formal occasions. Tuck in your shirt for most business occasions. Tuck in your shirt for events that require class. Leave your shirt untucked for casual fare.