Summer is almost here, and with the warmer weather comes an annual phenomenon: concert tour season. Acts new (Adele, Katy Perry) and old (Hall & Oates, Def Leppard) will be playing shows in the coming months, and they've selected a range of artists to open for them. Here, a look at the most unexpected main act-opening act pairings, and what the match-ups say about the headlining artists.