Minecraft wiki entry for Dual_wielding, associated text: The main hand always takes priority when trying to use an item. For example, if you had stone in your main hand and dirt in your off-hand, you will always place the stone. The item in the off-hand will only be used if the item in the main hand lacks a use, or if it's holding nothing. If the item in the main hand cannot be used due to certain circumstances, then the off-hand is used. For example, if you have a block equipped on the main hand and a snowball on the off-hand, face anywhere you cannot reach a block (e.g. into the sky), and then press use, the snowball will be thrown instead of placing the block. This is because the block cannot be placed anywhere, and so your off-hand is used instead.