So if you want to increase your chance of getting an item you need to kill monsters with higher mlvl than the TC of the desired item and with a mlvl higher than the ilvl/qlvl of the desired item.

If the mlvl is lower than the ilvl of the unique/set item it tried to spawn it will spawn as a rare with triple the amount of durability instead. Set and Unique items can have higher item level (ilvl, some times also refereed to as quality level/qlvl) than TC, before such an item can drop the mlvl needs to be equal to or higher than the ilvl. Once the item has been chosen a check for the quality of the item is done, this is where your Magic Find (MF) helps. Other items(ring, amulets, jewels, charms and runes) work a bit different, see the above links for more information on them. In short for weapons and armors when a monster dies or a chest is opened the item that drops is picked from a Treasure Class (TC), the TC has to be equal to or less than the monsters level(mlvl). There is a good guide here for how items are generated. Maximizing your chances of a specific item is a complex set of events when an item is generated.

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