Again, they will not work on Magic, Set, Unique or Rare items. This means that Set Items, Unique Items, or Magic Items with enough Sockets and the proper item type will not allow or convert to Rune Words. Rune Words will only work in Socketed non-magical items.

If the formula requires a sword, the formula will not work if you use an axe or spear.

The Socketed Item type must be the correct item type for the formula.

A 4 socket item will not work with a 3 socket formula. If the Rune Word is a 3 Rune formula then you must use an item with exactly 3 sockets. Socketed Items must have the exact number of sockets as the Rune Word formula requires.When you create a Rune Word you still receive all of the magical bonuses that were already on each Rune you Socketed.The benefits of using a Rune Word is that you can make the Rune Word's magical bonuses apply to any type of item allowed with the proper number of sockets.

If the player puts certain combinations of Runes in the correct order into an item with exactly that number of sockets and of the correct item type, the item's name will change into a 'unique' name, displayed in gold, and the item will acquire extra powers, depending on the 'rune word' that was used.

Diablo 2 skill and item calculator