Minecraft wiki entry for Dual_wielding, associated text: Arrows that are held on the off-hand take priority over arrows in any other slot when you shoot a bow. This allows you to fire a specific arrow by selecting it using your off-hand. Conveniently, you can do vice-versa and hold the bow on the off-hand with arrows selected on your main hand. This also allows you to fire a specific arrow by selecting it using your main hand.