<prompt> The bifurcation of trachea occurs most commonly atT3T5T7T9  <response>: The level of bifurcation of the trachea varies from the lower third of the fouh to the middle of the seventh thoracic veebrae. The average position is at the level of the lower third of the fifth thoracic veebra.Ref: PL Dhingra, Diseases of Ear, Nose Throat, 7th edition, pg no. 319 