Each Gloranthan culture uses somewhat different runes, though the Jrusteli God Learners found many similarities (for example, both the Dara Happans and the Orlanthi use a circle for the concept of Fire and a square for the concept of Earth). These similarities are accepted as proof of the underlying universality of the runes.
The Orlanthi assign each god one or more runes, representing the powers of that god. Some gods are said to "own" runes, meaning they have the strongest association with that abstract power. For example, Orlanth owns the Air rune, though other deities such as Urox and Valind also have the rune.
Not all runes are equally important -- for example, Cold and Shadow can be considered to be aspects of Darkness.
Note: The runes shown here are from a font created by Oliver Jovanovic (you can download it from the archive or from this site. The rune designs themselves are by Greg Stafford.
David Dunham Page