White Croats
It is considered that the Czech-Polish Croatian tribes were related to the Croatian tribes from Zakarpattia and Prykarpattia in Ukraine. They became separated during the migration period and seemingly formed one large Proto-Slavic tribe. It is theorized that the migration of those (White) Croatian tribes in the 7th century was the second and final Slavic migratory wave to the Balkans, which is related to the thesis by Bogo Grafenauer about the double migration of Slavs. According to this thesis, although it is possible that some Croatian tribes were present among Slavs in the first Slavic-Avar wave in the 6th century, it is argued that the Croatian migration in the second wave probably was not equally numerous to make a significant common-linguistical influence, while others considered they arrived in a significantly larger number. Zdenko Vinski and argued that the double migration thesis is supported by the rare findings of objects and ceramics of Prague-Korchak culture on the territory of Croatia, Bosnia and Herzegovina, and Serbia which were dated to the end of 6th and beginning of the 7th century (indicating first wave Slavs), and did not make a continuity with Slavic archaeological findings from the 8th-9th century, indicating assimilation. The Croatian tribes are also seen as a warrior group who assimilated into already present Slavs. In the territory of present-day Croatia they also gradually assimilated with the Pre-Slavic population because archaeology indicates a significant continuity of late antiquity population which have partially withdrawn to the mountains, coastal cities and islands.
Interslavic language
During the second half of the 19th century Pan-Slavic language projects were mostly the domain of Slovenes and Croats. In this era of awakening national consciousness, the Russians were the only Slavs who had their own state; other Slavic peoples inhabited large, mostly non-Slavic states, and clear borders between the various nations were mostly lacking. Among the numerous efforts at creating written standards for the South Slavic languages there were also efforts at establishing a common South Slavic language, Illyrian, that might also serve as a literary language for all Slavs in the future. Of special importance is the work of Matija Majar (1809–1892), a Slovenian Austroslavist who later converted to Pan-Slavism. In 1865 he published Uzajemni Pravopis Slavjanski ("Mutual Slavic Orthography"). In this work, he postulated that the best way for Slavs to communicate with other Slavs was by taking their own language as a starting point and then modifying it in steps. First, he proposed changing the orthography of each individual language into a generic ("mutual") Pan-Slavic orthography, subsequently he described a grammar that was based on comparing the five major Slavic languages of his days: Old Church Slavonic, Russian, Polish, Czech and Serbian. Apart from a book about the language itself, Majar also used it for a biography of Cyril and Methodius and for a magazine he published in the years 1873–1875, "Slavjan". A fragment in the language can still be seen on the altar of Majar's church in Görtschach. Other Pan-Slavic language projects were published in the same period by the Croatian Matija Ban, the Slovenes Radoslav Razlag and Božidar Raič, as well as the Bulgarian from Macedonia Grigor Parlichev – all based on the idea of combining Old Church Slavonic with elements from the modern South Slavic languages.