Around the horn refers to a double play started by the third baseman (position 5), who throws to the second baseman (position 4) for the first out, and then to the first baseman (position 3) for the second out -- a 5-4-3 double play. Also, after a strikeout, with no one on base and less than 2 outs, the catcher will throw the ball "around the horn" and back to the pitcher (except the shortstop is included in this tradition.)
