Ob chav kub txias txias, tseem hu ua duplex txias chav, yog nruab nrog ob lub txias txias, uas feem ntau yog siv rau sib xyaw cia txiv hmab txiv ntoo, zaub, nqaij thiab nqaij nruab deg.Siv tib lub zog siv nyob rau hauv tib cheeb tsam, nws tuaj yeem ua tau raws li ntau yam haujlwm xws li khaws cia cov khoom, cov khoom khov, thiab cov khoom tshiab.