The medieval period saw the development of literary metres indigenous to the Kannada language. These included the "tripadi" (three-line verse, in use from the 7th century), one of the oldest native metres; the "shatpadi" (six-line verse, first mentioned by Nagavarma I in "Chhandombudhi" of c. 984 and in use from 1165), of which six types exist; the "ragale" (lyrical narrative compositions, in use from 1160); the "sangatya" (compositions meant to be sung with a musical instrument, in use from 1232) and the "akkara" which came to be adopted in some Telugu writings. There were rare interactions with Tamil literature, as well. Though religious literature was prominent, literary genres including romance, fiction, erotica, satire, folk songs, fables and parables, musical treatises and musical compositions were popular. The topics of Kannada literature included grammar, philosophy, prosody, rhetoric, chronicles, biography, history, drama and cuisine, as well as dictionaries and encyclopedias. According to critic Joseph T. Shipley, over fifty works on scientific subjects including medicine, mathematics and astrology have been written in the Kannada language. Kannada literature of this period was mainly written on palm leaves. However, more than 30,000 more durable inscriptions on stone (known as "shilashasana") and copper plates (known as "tamrashasana") have survived to inform modern students of the historical development of Kannada literature. The Shravanabelagola inscription of Nandisena (7th century), Kappe Arabhatta inscription (c. 700), and the Hummacha and Soraba inscriptions (c. 800) are good examples of poetry in "tripadi" metre, and the Jura (Jabalpur) inscription of King Krishna III (964) is regarded as an epigraphical landmark of classical Kannada composition, containing poetic diction in "kanda" metre, a form consisting of a group of stanzas or chapters. Elegiac poetry on hundreds of "veeragallu" and "maastigallu" (hero stones) written by unknown poets in the "kanda" and the "vritta" (commentary) metre mourn the death of heroes who sacrificed their lives and the bravery of women who performed "sati".