Runes are small stones inscribed with runic characters which can be placed in any socket of an item to add modifiers to that item in Expansion games. The modifier(s) added can vary depending on whether a rune is placed in a socket in a weapon, helm and body armor, or shield:
Once placed in a socket, a rune cannot be removed without destroying it. Runes can also be used in combination to make Rune Words, as well as being required to make crafted items and make or manipulate other items using the Horadric Cube (including upgrading lower runes to higher runes). They can normally only be dropped by monsters, and their required level normally reflects their rarity (becoming rarer as rlvl increases). Monsters spawned in games with more players are normally less likely to have no drop and thus more likely to drop items (including runes), but Better Chance of Getting Magic Items has no effect on the quantity or quality of runes dropped. Each rune fills one inventory space and they cannot be stacked.
The division between runes of low, medium and high rarity is convenient rather than exact: for example, The Countess has a much higher chance of dropping runes than similar Unique or Super Unique monsters (up to an Ist Rune in Hell), while the reward for destroying Mephisto's Soulstone on the Hellforge is a much higher chance of a medium or high rune (1/11 chance each of a Mal Rune, Ist Rune or Gul Rune in Hell).
Runes cannot be bought from vendors in town unless another player sells them and at least one player remains in town afterwards to prevent the inventory being reset. If runes are sold to a vendor, they are displayed under the Misc tab of the inventory if there is sufficient space for them. Gold from selling runes to vendors in Hell is as follows:
However, selling runes (particularly those rare enough to sell for maximum 35,000 gold in Hell) is not recommended as they are not as plentiful as gold.