Minecraft wiki entry for Beginner%27s_guide, associated text: From the start of the game, you can see nine special inventory slots, called your "hotbar", but you also have more slots which are normally hidden. As you pick up items, the first few appear in your hotbar slots, but once those are full, they go into the 27 slots of your main inventory. At any given time, one of your hotbar slots is "selected", and the item in that slot is considered to be "in hand". (You can see your item in hand in front of you.) You can press keys 1 through 9 (or use a mouse-wheel if you have one) to choose which hotbar slot is active, thus you can quickly switch among up to 9 handy tools or other items.