Six Dynasties poetry refers to those types or styles of poetry particularly associated with the Six Dynasties era of China (220 CE – 589 CE). This poetry reflects one of the poetry world's more important flowerings, as well as being a unique period in Classical Chinese poetry, which, over this time period, developed a poetry with special emphasis on romantic love, gender roles, and human relationships. The Six Dynasties era is sometimes known as the "Age of Fragmentation", because China as a whole through this period lacked unification as a state, at least for any extended period of time; and, instead, many states rose and fell, often overlapping in existence with other states. Which of the various states and dynasties constituted the "6" dynasties of the Six Dynasties period varies somewhat according to which of the traditional selection criteria are chosen. The Six Dynasties era covers several somewhat overlapping main periods including all of the following: the Three Kingdoms (220–280), Jin dynasty (266–420, in 2 approximate parts, Western Jin 266–316 and Eastern Jin 317–420), the Sixteen Kingdoms (also known as the "Sixteen States", 304 to 439), and the Southern and Northern Dynasties (420–589). Sometimes, chronological discrepancies occur in regard to the turbulent political events of the time, from which these traditional historical-era designations derive, together with the somewhat different chronology of poetic (versus political) developments. Thus, neither the lives of the poets nor the trends in their poetry fit gently and neatly together with these period dates. Furthermore, conversions to the Common Era dating system can create further complications. However, regardless of the chronological difficulties, major developments of poetry during the Six Dynasties include formalizing the distinction between the Jian'an era regular yuefu and the shi style poetry, further development of the fu, theoretical work on technique, and the preservation of both Six Dynasties and earlier poetry by collecting and publishing many of the pieces which survive today into various anthologies consisting all or in part of poetry.