Theodor Nöldeke's chronology is based on the assumption that the style of the Quran changes in one direction without reversals. Nöldeke studied the style and content of the chapters and assumed that first, later (Madinan) chapters and verses and are generally shorter than earlier (Meccan) ones, and second, that earlier Meccan verses have a distinct rhyming style while later verses are more prosaic (prose-like). According to Nöldeke, earlier chapters have common features: many of them open with oaths in which God swears by cosmic phenomena, they have common themes (including eschatology, creation, piety, authentication of Muhammad's mission and refutation of the charges against Muhammad), and some Meccan chapters have a clear 'tripartite' structure (for example chapters 45, 37, 26, 15, 21). Tripartite chapters open with a short warning, followed by one or more narratives about unbelievers, and finally address contemporaries of Muhammad and invite them to Islam. On the other hand, Madinan verses are longer and have a distinct style of rhyming and concern to provide legislation and guidance for the Muslim community.