Your backpack (in some games called your inventory) contains all items you possess which are not already worn (if an item in your backpack is worn it will not take up a backpack item space until it has been removed from your body) or deposited in the warehouse. You open up the backpack window by pressing the B key on your keyboard. It also displays how much money you currently have on you not counting any you have deposited in the warehouse or in the mail unclaimed.
Items that are in your backpack can have their icons dragged to the hotbar keyslots for easy future access if you so desire. Items in your backpack can also be used or worn directly from this interface by simply right clicking them. Left clicking the item allows you to select it for dragging to another location.
Every time you gain a new item of the same type it will be added to the stack. If you had 1 small mana pill and buy 10 new ones instead of taking 2 slots of item space it will only take one spot still and the item's number at the bottom right of the icon will rise to say 11 instead of 1. Once a stack reaches 99 it is full so once you gain a hundredth item of that type a new stack for the same item will begin taking up a second item space in your backpack, and this will occur again once that stack reaches a full 99 as well.
The exception to the stacking rule occurs for equipment, that means your weapon and armor pieces or anything else that becomes visible once you open up the equip window. These equipment type items will always take up 1 item space per 1 piece.
Once your item space is all used up you will be unable to pick up new items or accept trade items with another player until you gain at least 1 empty space, even if the item you're trying to receive is one from a stack you've already started and is almost an empty stack.
--Mikuri 03:35, 6 March 2008 (UTC)