A stack is a group of identical items that can be stored in a single inventory space. If an item is stackable, up to 250 copies of it can be held in one stack. The total number of items in a stack is shown as a yellow number over the top of the item's icon.
If you have two stacks of an item, you can combine them by dragging one over the other; if there are more than 250 units total, the stack dragged into will be filled and overflow will remain in the original stack. A stack can be broken by holding the shift key while dragging: a dialog will appear when you place the item allowing you to specify the new stack size.
When items are automatically added to a container — by picking them up off the ground, receiving them in a trade, or dragging them onto a bag icon or storage tab — partial stacks of that item in the container will be filled before an empty inventory slot is taken.
 Stackable items
- Common and rare crafting materials.
- Dyes of the same color.
- Keys and Lockpicks.
- Certain quest items.
- Experience and passage scrolls.
- Skill tomes of the same profession and rarity.
- Trophies, including reward trophies for quests, such as Monastery Credits, and festivals, such as Lunar Tokens.
- Hero armor upgrade items.
- Salvage items that do not represent armor, such as hides and webs.
- Zaishen Coins of the same type.
- Single-use consumables of the same type.
- When the items are taken from the inventory automatically (for example, when selling materials to traders) and you have more than one stack of such items, the topmost stack will be used first.
- In most cases, interface mechanics is able to take the items from several stacks in inventory at once; however, rarely this operation can result in failure (see a bug with Votive Candle). It's recommended to merge items in a maximum possible stack before trading them to NPC.