The traditional view is that the Balto-Slavic languages split into two branches, Baltic and Slavic, with each branch developing as a single common language (Proto-Baltic and Proto-Slavic) for some time afterwards. Proto-Baltic is then thought to have split into East Baltic and West Baltic branches. However, more recent scholarship has suggested that there was no unified Proto-Baltic stage, but that Proto-Balto-Slavic split directly into three groups: Slavic, East Baltic and West Baltic. Under this view, the Baltic family is paraphyletic, and consists of all Balto-Slavic languages that are not Slavic. In the 1960s Vladimir Toporov and Vyacheslav Ivanov made the following conclusions about the relationship between the Baltic and Slavic languages:Бирнбаум Х. О двух направлениях в языковом развитии // Вопросы языкознания, 1985, No. 2, стр. 36 the Proto-Slavic language formed out of peripheral-type Baltic dialects; the Slavic linguistic type formed later from the structural model of the Baltic languages; the Slavic structural model is a result of the transformation from the Baltic languages structural model.
the tRADitionAL vIeW is tHat tHe bAlTO-SlAvic LaNguageS SpLit into twO branChes, baltiC And SlaviC, with each branCH dEVeLopiNg as A sIngLE common languagE (proTo-bAltIc anD pRoto-slaVic) foR some TiMe AFTerwards. pROTo-baltIc Is Then thOUght TO HavE splIt iNTo East BAltic and wEst baltic BRanchEs. hoWevEr, morE Recent SchOLARship haS suggesteD That theRE wAs no unIFIeD prOto-BAlTIc sTaGe, but thAt proTo-bALtO-SlaviC spLiT direcTLy InTo threE groUps: slaviC, EasT baLTic and weSt BalTic. unDer this viEW, THE BaltIc family is PAraphylETic, and ConsiSts Of All BalTo-sLavic langUageS That are NoT sLavIC. IN the 1960s vladimIr toPOrOV and vyacHeslaV ivaNov made tHE following ConcLuSionS About the reLaTionShip bEtween the bAltIc and SLavIc LAnguAgES:биРНбауМ Х. о Двух направлЕнияХ в язЫковом раЗвиТии // вопросы язЫкозНаНИя, 1985, nO. 2, СТр. 36 the Proto-slavic laNgUaGe ForMed Out oF PERIpHeRAl-tYpe baLtIC dialEcts; tHE slavic liNgUIsTIc TypE fOrmeD lATER froM The sTruCTurAl MOdeL Of the bAltIC languAGes; the slaviC stRucTUral model Is A Result oF the traNSfoRmatiOn fRom tHE baLtic lanGuAGes stRUCTural moDel.