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=What is a Treasure Class?=

Treasure Classes are groupings of items with similar levels (Qlvl), one of which is picked to drop once a given TC is picked to drop from.  There are 30 Weapon and Armor TCs, starting at 3, and going by 3's up to 90.  Each TC contains all of the items of that type (weapons or armor) that have a level between that TC and the next lower one.  For example Weapon 27 contains all weapons that are level 25, 26, and 27.  Armor 27 contains all armors of level 25, 26, and 27. Weapon 30 contains all weapons that are lvl 28, 29, and 30.  And so on. All monsters that can drop a given Armor can drop the same # Weapon TC.  So if it can drop Weapon 84, it can drop Armor 84, though the odds for weapon or armor are often different from the same monster.

Every monster has a range of assigned TCs it may drop from, always starting at 3, and going upwards as the monsters get higher level later in the game. For a rule of thumb, most monsters top TC is about the same as their Mlvl, or slightly lower.  So an Mlvl 70 monster's top TC would be 66 or 69, most likely. Everything can drop from Weapon or Armor 3, and the misc TCs, but hardly anything can drop from TC 90.

The TC selection is made when the monster dies, if it picks to drop an item.  All TCs are checked at the same time, with various percentages assigned to every TC, depending on which monster or chest is dropping the item.  Once a TC is selected the game selects an item from that TC, and then determines the quality of the item, factoring in your MF for that portion.  MF has no effect on the TC or item from the TC selection. 

All TCs are weighted, in terms of how likely a monster is to drop from them.  Monsters are always very unlikely to drop from their highest possible TCs, and grow more likely to drop from a TC as you move down in number (there are some slight exceptions). If a monster can drop at most TC 84, the odds of it doing so are very poor.  It would have better odds at dropping from TC 81, better than that from TC 78, better yet at TC 75, etc.  So if you want to find an item from TC 84, generally speaking you're more likely to get it killing something that can drop all the way up to TC 90, than you are if you kill something that can only drop TC 84.

Here's a quick example of three popular monsters on hell difficulty.  See the Monsters TC page for more tables like this. 


{|
! Monsters
! Armor 90
! Armor 87
! Armor 84
! Armor 81
|-
| Pindleskin
| 1233 to 1
| 308 to 1
| 221 to 1
| 218 to 1
|-
| Baal
| NA
| 989 to 1
| 124 to 1
| 97 to 1
|-
| Shenk
| NA
| NA
| 2339 to 1
| 324 to 1
|}

As you can see, Pindleskin is the only one of these 3 that can drop from the highest TCs, Weapon and Armor 90, and his odds to do so are very poor, which is why items in TC 90 are virtually impossible to find. If you want something from TC 84, such as Lacquered Plate, the set version of which is Tal Rasha's Guardianship, Shenk can possibly drop it, but his odds to do so are tremendously poor. Just 1 out of every 2339 items he drops will be from Armor 84, and out of that he has just a 3/17 chance of dropping a Lacquered plate, and then your odds of it being a set item are figured from there.

Meanwhile, Pindleskin has far better odds of dropping something from Armor 84, over 10x better than Shenk's odds. Baal has even better odds, but takes longer to reach, which is why you need to figure more than just the straight odds when determining which monsters you want to spend your time killing.

A few other basic terms you should know.