The Mystery Age Jokka used two systems of writing, a phonetic and a logographic. The logographic system has fallen out of use, but the phonetic survived and has changed very little since that time.
Allowed consonants: p t k b d g f th s sh h dz m n ñ dl l r w
Allowed vowels: a (ah), e (eh), oi (dipthong), o (oh), u (ooh), i (ih), ii (ee)
Paper is very expensive, even in Het Kabbanil. Most Jokka use stone tablets or wooden plates treated with wax so they can be wiped and re-used. Some use colored bowls and leave messages for one another with different tokens.
Information here about the custom of setting out stories/histories/etc.