A DLT is made up of two small-lumen endotracheal tubes of unequal length fixed side by side. The shorter tube ends in the trachea while the longer one is placed in either the left or right bronchus in order to selectively ventilate the left or right lung respectively. The first double-lumen tube used for bronchospirometry and later for one-lung anaesthesia in humans was introduced by Carlens in 1949.   Modifications to the original Carlens tube have been introduced by White,  Robertshaw  and others. The most commonly used DLTs today are the Carlens and the Robertshaw tubes.  These allow single-lung ventilation while the other lung is collapsed to make Thoracic surgery easier or possible. This may be necessary so as to facilitate the surgeon's view and access to relevant structures within the thoracic cavity . The deflated lung is re-inflated as surgery finishes to check for leakages or other injuries .