When attempting to throw out a runner at second base who takes the throw?

That would depend on 'who' is trying to throw the runner out. This answer will assume the questioner is asking about a stolen base and the 'who' is the catcher. The defensive player that covers second base on a steal attempt is the player that the defense believes is on the side of the field that the ball will NOT be hit to. Factors that go into this decision are whether the batter is righthanded or lefthanded, where the pitcher is going to pitch the ball (inside or outside), and how good of a batter the player is (whether they can hit the ball to all fields, whether they are a pull hitter). Generally, if the batter is righthanded and the pitcher is going to pitch inside the odds are better, if the batter makes contact with the ball, that it will be hit to the left side of the field. In this case, it will be the second baseman's responsibility to cover the base if the runner attempts to steal. If the batter is lefthanded and the pitcher is going to pitch inside the odds are better, if the batter makes contact with the ball, that it will be hit to the right side of the field. In this case, it will be the shortstop's responsibility to cover the base if the runner attempts a steal. The decision of who will cover second base on a steal attempt is made before every pitch. On one pitch the second baseman may be the player to cover and on the next pitch the shortstop may be the player to cover. If you are at a game, you may notice when a player is on first base that the second baseman will put his glove up to his face and look at the shortstop just before a pitch is made. The second baseman is giving a signal to the shortstop as to who will cover second base on a steal attempt. Usually, the signal is an open mouth or closed mouth. If the second baseman looks at the shortstop and his mouth is open, this means the second baseman will be the player to cover if a steal is attempted; if the mouth is closed the shortstop will cover. Or vice versa ... it all depends on how the team has set up their signals.