Minecraft wiki entry for Inventory, associated text: Stacks can be grabbed by pressing left-click on the stack and split by pressing right-click. If the stack is an odd quantity, then it leaves the smaller half (for example, a stack of 7 leaves 3 items in the slot and grabs 4). Conversely, while holding a stack of items in the inventory, left-clicking places the full stack in the slot, and right-clicking place just one item. Double-clicking an item grabs up to a full stack from all the items within the inventory.