The Letter Sticks were "long rods of ash wood carved with spells" inscribed in "ancient symbols".
Dallben the Enchanter would consult Hen Wen, the oracular white pig, through the use of the sticks, but only in a matter of greatest urgency. The great enchanter attempted to do so during the rise of the Horned King, but Hen escaped before Dallben could retrieve the rods; and once, successfully, later after Dyrnwyn had been stolen by Arawn.
The Letter Sticks were prophetic devices. Dallben would set them upright in the soil of Hen Wen's enclosure, and Hen -- if she were feeling unafraid -- would walk up to various rods in turn, while Dallben scribbled the meaning of each selection on a bit of parchment.
Hen might need gentle encouragement from an Assistant Pig-Keeper, assuming one was handy, to complete the prophecy, especially when the matter was one of perilous impact.
Instances of Use
The first time Dallben wished to use them within Taran's seeing was during the events of The Book of Three, but the enchanter could not find the rods before Hen Wen escape her enclosure and fled Caer Dallben in a panic. The sole instance Taran saw the Letter Sticks used was at the outset of The High King. At that time, Hen selected a prophecy concerning the stolen sword Dyrnwyn, but the rods exploded before she could complete the final line.