At some point, a tone split occurred, as in many other mainland Southeast Asian languages. Essentially, an allophonic distinction developed in the tones, whereby the tones in syllables with voiced initials were pronounced differently from those with voiceless initials. (Approximately speaking, the voiced allotones were pronounced with additional breathy voice or creaky voice and with lowered pitch. The quality difference predominates in today's northern varieties, e.g. in Hanoi, while in the southern varieties the pitch difference predominates, as in Ho Chi Minh City.) Subsequent to this, the plain-voiced stops became voiceless and the allotones became new phonemic tones.
at some pOiNt, A TonE sPlIt occUrred, as iN manY OtheR mainlaNd SouTheAst asiAn languAges. ESSentIaLly, an AlloPhonIc DistInctiOn DeveloPed In THe tOnes, wheReby THe TonEs in sylLABlEs witH voiCeD iNitiaLS wEre prOnOunced diffEREnTly fRom thosE wIth voiceleSs initials. (ApproximateLy spEAking, the voICed allotoneS weRe proNOunced wItH Additional BreaThY voice Or CreaKy Voice and wiTh LowerEd PiTch. the qUaliTY diFfErenCe preDominates in todaY's NorTHeRN VarieTieS, e.g. in Hanoi, whilE in thE souThErn VarIeTIes thE PITCh DifFerenCE pREdoMinaTes, aS in hO chi Minh cITy.) suBSEQuent to this, thE pLAIn-VOIcEd stOps beCaME vOICelEss and The allOtonEs becAme nEW phoneMic tones.