My apologies if this has already been asked, but I could not seem to find it. If I understand correctly, a pitcher cannot throw to an unoccupied base without stepping off the rubber. Doing so constitutes a balk if there is a runner on base. I witnessed a situation today where there was a runner on 1st and 2nd. After the pitcher had come set and was preparing to make his delivery, the running on 2nd started for third base. The pitcher was a right-handed thrower, and was therefore facing third base. When he heard that the runner was going and noticed that it was the runner on 2nd and was going to 3rd, the pitcher threw to 3rd base. The runner was tagged and called out. I was under the presumption that, technically, the runner is not on 3rd, so 3rd is unoccupied. Therefore, by throwing to 3rd, is that not a balk? I figured that, if the pitcher did not step off the rubber, he would have to either throw home or spin and throw to 2nd. Am I wrong? Is the pitcher allowed to throw to 3rd in this situation? Or, am I correct and a balk should have been called on the pitcher? Thank you for your time.