Also present in Balto-Slavic were the diphthongs *ei and *ai as well as liquid diphthongs *ul, *il, *ur, *ir, the latter set deriving from syllabic liquids; the vocalic element merged with *u after labiovelar stops and with *i everywhere else, and the remaining labiovelars subsequently lost their labialization. Around this time, the PIE aspirated consonants merged with voiced ones:  Once it split off, the Proto-Slavic period probably encompassed a period of stability lasting 2,000 years with only several centuries of rapid change before and during the breakup of Slavic linguistic unity that came about due to Slavic migrations in the early sixth century. As such, the chronology of changes including the three palatalizations and ending with the change of *ě to *a in certain contexts defines the Common Slavic period. Long *ē and *ō raised to *ī and *ū before a final sonorant, and sonorants following a long vowel were deleted. Proto-Slavic shared the common Balto-Slavic merging of *o with *a. However, while long *ō and *ā remained distinct in Baltic, they merged in Slavic (after the previous change), so that early Slavic did not possess the sounds *o or *ō. A tendency for rising sonority in a syllable (arrangement of phonemes in a syllable from lower to higher sonority) marks the beginning of the Common Slavic period. One aspect of this, generally referred to as the "Law of Open Syllables", led to a gradual elimination of closed syllables. When possible, consonants in the coda were resyllabified into the onset of the following syllable. For example, *kun-je-mou "to him" became *ku-nje-mou (OCS "kъňemu"), and *vuz-dā-tēi "to give back" became *vu-zdā-tēi (OCS "vъzdati"). This entailed no actual phonetic change, but simply a reinterpretation of syllable boundaries, and was possible only when the entire cluster could begin a syllable or word (as in *nj, *zd, *stv, but not *nt, *rd, *pn).