In NetHack, many varieties of items can stack into one inventory slot. This includes:
- gems, glass, rocks, and gray stones
- comestibles, except for hatchable eggs and revivable corpses
- candles, if close in age and type
- certain ranged weapons:
In order for objects to stack, their type, beatitude, enchantment, erosion status, and identification status must all be exactly the same. For example, if you have two daggers, and only drop one on an altar and see it was uncursed, those two daggers will no longer stack (because one is identified as uncursed, and the other's BUC status is not formally identified). Then, if you were to go a step further, and use a scroll of identify on the dagger you didn't drop, and it was an uncursed +0 dagger, then it would still not stack with your plain "uncursed dagger" (because only one dagger's enchantment is identified).
The usual way to split a stack is to supply a numeric parameter to the adjust, drop or pick up command, or when moving items into or out of a container. For example, if you have 7 daggers in inventory slot a, you can type d 4 a to drop 4 of them.
Besides conserving inventory space, getting items to stack is important for efficiency when using holy water to bless stacks of items, or using a scroll of enchant weapon to enchant an entire stack of arrows. It can also affect how many projectiles are fired in a single turn; a player can only get off multiple shots in one turn if the projectiles are from the same stack.
An item being picked up or removed from a container will automatically stack with another item of the same type in your inventory, if it can. If you want to prevent this, you will have to #name the individual items with different names. It is not sufficient to split the stack into two and name one of them - stacks will always take on a name if they do not already have one. This phenomenon can often be observed early on, when one finds an orcish dagger, has the pet BUC test it, and then names it uncursed, only to see other new daggers stack with it.
As of NetHack 3.6.1, you can wield a stack of throwable weapons (e.g. daggers or spears) and then quiver them. This will give you the option to leave one weapon wielded and quiver the rest, which will split the stack accordingly.
Splitting stacks is useful when someone needs to split up a stack of water to make an initial unholy water by reading while confused a blessed scroll of remove curse. It may be tedious, but it is probably wise to split up around 10 stacks for this purpose.
Getting potions to stack is of the utmost importance in alchemy. All alchemy results will be "diluted potion of X", and diluted potions will only stack with other diluted potions. Because of this, later on in the game it can be advantageous to dilute (by dipping once) potions for alchemy, for increased efficiency. Just beware all the risks associated with dipping and fountains, and take care to have identified acid, as dipping acid will cause an explosion.