North Slavic languages

The term North Slavic languages has three meanings.
North versus South Slavic.
It is sometimes (albeit not very often) used to combine the West Slavic and the East Slavic languages into one group due to the fact that the Southern Slavic dialects were geographically cut off by the Hungarian settlement of the Pannonian plain in the 9th century along with Austria and Romania being geographical barriers, in addition to the Black Sea. Due to this geographical separation, the North Slavs and South Slavs developed independently of each other with noteworthy cultural differences.
North Slavic peoples today include the Belarusians, Czechs, Kashubians, Poles, Silesians, Rusyns, Russians, Slovaks, Sorbs, and Ukrainians. They inhabit a contiguous area in Central and Eastern Europe and North Asia.
The greatest disparities are between South Slavic tongues and the rest of the Slavonic languages. 
Moreover, there are many exceptions and whole dialects that break the division of East and West Slavic languages; thus the Slavs are clearly divided into two main linguistic groups: the North Slavs and the South Slavs, which can then be further categorised as the Northwest tongues (Czech, Kashubian, Polish, Silesian, Slovak, and Sorbian) and the Northeast ones (Belarusian, Russian, Rusyn, and Ukrainian) – whereas the Southern branch is split into the widely accepted groups of the Southwest languages (Serbo-Croatian and Slovene) and the Southeast tongues (Bulgarian and Macedonian). This model is argued as being more appropriate than the triple dissection of east, west and south.
Much overlap can be found between the Northwest and Northeast branches. Ukrainian and Belarusian have both been hugely influenced by Polish in the past centuries due to their geographic and cultural proximity, as well as some efforts at Polonising the Ruthenian population of the Polish Commonwealth. Professor Michał Łesiów once said that "there are no two languages in the Slavic area that were as equally close to each other as Polish and Ruthenian".
According to Kostiantyn Tyshchenko, Ukrainian shares 70% common vocabulary with Polish and 66% with Slovak, which puts them both ahead of Russian (at 62%) in their lexical proximity to Ukrainian. 
Furthermore, Tyschenko identified 82 grammatical and phonetic features of the Ukrainian tongue – Polish, Czech and Slovak share upwards of 20 of these characteristics with Ukrainian, whereas Russian apparently only 11.
In contrast to other dialects of Slovak, Eastern dialects (sometimes called Slovjak) are less intelligible with Czech and more with Polish and Rusyn.
An extinct branch of Slavic.
 suggested that a separate, now extinct, branch of North Slavic languages once existed, different from both South, West, and East Slavic. The dialect formerly spoken in the vicinity of Novgorod (the Old Novgorod dialect) contains several Proto-Slavic archaisms that did not survive in any other Slavic language, and may be considered a remnant of an ancient North Slavic branch.
Constructed North Slavic languages.
There is a group of artistic languages forming a "fictional" North Slavic branch of the Slavic languages. The best-known examples of constructed North Slavic languages are: Sevorian ("Sievrøsku"), Nassian ("Nassika"), Seversk, Slavëni, and Vozgian.