A special case in the form of Baltic and Slavic collective numerals. In Proto-Indo-European, there were thematic outcomes for "three" and “four”:
*trei̯om~*troi̯om from
*trei̯es (compare the Sanskrit
trayam) and
*kʷetu̯erom~*kʷetu̯orom from
*kʷetu̯ores (compare the Sanskrit
catvaram). In Balto-Slavic they have given e. g. the modern Lithuanian
treji : Russian
трое/troje and
ketveri : четверо/četvʲero, and hence
-j- has spread to “two” producing
dveji : двое/dvoje (back in PIE: compare the Sanskrit
dvayam) and
-er- spread to the numerals above “four” producing
penkeri : пятеро/pʲatʲero, šešeri : шестеро/šestʲero and so on (the Russian
сторица/storʲiʦa implies the former existence of even *
съторо/sъtoro).