When you're wearing a suit, you don't want all the junk in your pockets to create unsightly bulges. This diagram shows you the perfect spots to keep your stuff handy without ruining the look.
This image, from advice site Art of Manliness, demonstrates the ideal location for all of the stuff you're likely to carry around while wearing a suit. Your phone and a slim wallet, it suggests, should go in your inner breast pockets, while more awkwardly-shaped items like keys and a larger wallet should be in the lower pants pockets.
If you want to add a pocket square, pen, or handkerchief, there's a right place for everything there, too. Of course, this all assumes you have these pockets. Not every designer takes pockets into consideration, but if you have them, make sure you use them well.
Where to Put Your Stuff In Your Suit [The Art of Manliness]