It depends. In little league, the shortstop is the rule. Usually in high school, the shortstop will cover for a left handed hitter, and the second baseman will cover for a righty. In college and the pros, it's up to the shortstop to call it, and that will depend on the hitter and the called pitch. If the shortstop wants to cover the throw he'll usually show the second baseman a closed mouth (for MMMe), and if it's to be the second baseman, the shortstop will show him an open mouth (you). eg. a lefthanded inside hitting batter, with a pitch called for low and outside; shortstop shows the second baseman an open mouth "yOU" - second baseman covers second so the shortstop can field the more likely left side hit. Of course, he'll shield this from the batter, using his glove.