What are the cheapest airlines in the US?

What are the largest or most significant US airlines to have gone bankrupt, become defunct, or been acquired?

• I would ask for more specifics. Delta, now the world's largest airline has gone bankrupt and exited in 2007. Eastern Airlines, once having more daily departures than Delta, went defunct in 1991. Both Continental and Northwest Airlines were acquired. To my knowledge, these would be the largest acquisitions.

• Answer:

  Somewhere on the net I've seen a family tree for airlines in the US but I can't remember where or find it easily.   I did find this which shows how UA and CO grew up (and went bankrupt) http://farm5.static.flickr.com/4112/4841802863_fdac8cd65d_b.jpg and this for DL/NW http://deltamuseum.org/pdf/Delta-NW_Family_Tree.pdf Otherwise my memory may be hazy but... Western folded into Delta Pan Am lost the plot and sold various route rights to UA and DL but before that Pan Am bought National TWA was taken over by AA