Accordingly, there were two phases in Early Buddhism: 1. ‘Samyutta/Samyukta Buddhism’ based on saṃyukta-kathā 相應教, and 2. ‘Nikaya/Agama Buddhism’ based on the four principal Nikayas/Agamas (according to Ven. YinShun’s The Formation of Early Buddhist Texts ).
However, the extant Nikayas/Agamas are sectarian texts. One can seek an understanding of early Buddhist teachings by studying them comparatively.