Incidentally, my preceptor Ayya Gunasari once asked me to make a short English summary of Oskar von Hinüber’s German study " The nine angas, An early attempt to classify Buddhist texts".
I’m pasting it below. Bhante Sujato has also written about how the nine angas were transformed into the four nikayas in „A history of mindfulness“, pages 19-130.
The nine angas
An early attempt to classify Buddhist texts
Oskar von Hinüber
Tries to explain how the old classification of the nine angas was replaced by the well-known 5 nikayas.
2 difficulties: 1) It’s unclear what some of these angas mean; 2) The angas are heterogenous and seem to refer to different categories of texts (Itivuttaka & Jataka vs. Sutta & gatha vs. Abbhutadhamma & vedalla)
The anga-system goes back to the earliest times of Buddhism, however it seems to have fallen out of use very fast, since even early inscriptions speak of the later Tipitaka-system.
AN III/237, 14-19 mentions a group of four of the nine angas (sutta, geyya, veyyakarana, abbhutadhamma). This seems to be earlier than the group of nine, due to lexicon and waxing-syllable principle.
Hypothetical meaning of these four:
sutta: patimokkha
geyya: verse texts
veyyakarana: prose texts / discourses
abbhutadhamma: Buddha legend
List of four was then expanded over time to nine
Of the rest of the nine, gatha refers to an entire genre of literature (verses), udana, itivuttaka, jataka are titles of texts in the Khuddaka Nikaya, vedalla is of unknown meaning.
But over time, the body of texts grew too big, so the anga system was abandoned and the nikaya system and the tipitaka came into use.
Mat, like Ayya Vimalanyani already mentioned, I’d really recommend to read Bhante Sujato’s “A History of Mindfulness”. In the first section he is outlining the development of the canon in general, whereas the second part is more specifically dedicated to the various Satipatthana texts. Most interesting!
One place I’ve come across this categorization scheme, where it’s used a lot, is in The Fundamental Teachings of Early Buddhism by Choong Mun-keat (all based on theories by Yin Shun).
The book categorizes all sections of the SN by using the first three components of this list: sutta, geyya and veyyakarana. He believes these three categories were the first to be developed in the SN/SA with other categories developing after that:
According to Yin Shun, by expansion of the three angas (compiled in SN/SN) new categories or classifications (angas) then came to exist: angas 6. Ityuktaka, 7. Jataka, 8. Vaipulya, and 9. Adbhuta-dharma were compiled in MA, DA, and EA; anga 4. Gatha corresponds to certain texts of Sutta-nipata, and 5. Udana is Dhammapada, both compiled in the Khuddaka-nikaya rather than being made part of the four basic Agamas/Nikayas